[doc] Fix typos

[klausler]

Thank you very much for fixing these problems!

[YangZhihui]

In D87672#2274896, @klausler wrote:

Thank you very much for fixing these problems!

Thanks，
may be you can help commit it : )
by the way, why the unittest failed?
This change should not fail the unit test.

[DavidTruby]

@YangZhihui if you don't have commit access I can commit this for you. Could you let me know the name and email address you'd like it committed under please? Thanks!

[DavidTruby]

Did this change ever get committed?

[YangZhihui]

In D87672#2500934, @DavidTruby wrote:

Did this change ever get committed?

Yes.
Please ignore it.

flang/docs/Extensions.md

* Classic C-style /*comments*/ are skipped, so multi-language header
   files are easier to write and use.
 * $ and \ edit descriptors are supported in FORMAT to suppress newline
   output on user prompts.
 * REAL and DOUBLE PRECISION variable and bounds in DO loops
 * Integer literals without explicit kind specifiers that are out of range
   for the default kind of INTEGER are assumed to have the least larger kind
   that can hold them, if one exists.
 * BOZ literals can be used as INTEGER values in contexts where the type is
-  unambiguous: the right hand sides of assigments and initializations
+  unambiguous: the right hand sides of assignments and initializations
   of INTEGER entities, and as actual arguments to a few intrinsic functions
   (ACHAR, BTEST, CHAR).  But they cannot be used if the type would not
   be known (e.g., `IAND(X'1',X'2')`).
 * BOZ literals can also be used as REAL values in some contexts where the
   type is unambiguous, such as initializations of REAL parameters.
 * EQUIVALENCE of numeric and character sequences (a ubiquitous extension)
 * Values for whole anonymous parent components in structure constructors
   (e.g., `EXTENDEDTYPE(PARENTTYPE(1,2,3))` rather than `EXTENDEDTYPE(1,2,3)`

 * `T` and `F` as abbreviations for `.TRUE.` and `.FALSE.` in DATA (PGI/XLF)
 * Use of host FORMAT labels in internal subprograms (PGI-only feature)
 * ALLOCATE(TYPE(derived)::...) as variant of correct ALLOCATE(derived::...) (PGI only)
 * Defining an explicit interface for a subprogram within itself (PGI only)
 * USE association of a procedure interface within that same procedure's definition
 * NULL() as a structure constructor expression for an ALLOCATABLE component (PGI).
 * Conversion of LOGICAL to INTEGER in expressions.
 * IF (integer expression) THEN ... END IF  (PGI/Intel)
-* Comparsion of LOGICAL with ==/.EQ. rather than .EQV. (also .NEQV.) (PGI/Intel)
+* Comparison of LOGICAL with ==/.EQ. rather than .EQV. (also .NEQV.) (PGI/Intel)
 * Procedure pointers in COMMON blocks (PGI/Intel)
 * Underindexing multi-dimensional arrays (e.g., A(1) rather than A(1,1)) (PGI only)
 * Legacy PGI `NCHARACTER` type and `NC` Kanji character literals
 * Using non-integer expressions for array bounds (e.g., REAL A(3.14159)) (PGI/Intel)
 * Mixing INTEGER types as operands to bit intrinsics (e.g., IAND); only two
   compilers support it, and they disagree on sign extension.
 * Module & program names that conflict with an object inside the unit (PGI only).
 * When the same name is brought into scope via USE association from