[docs][llvm] Fix typos

[SuJunda]

Summary

I don't have commit access.
Please help me commit it.
Thanks : )

[Paul-C-Anagnostopoulos]

LGTM. I will figure out how to commit this for you. Thanks!

[Paul-C-Anagnostopoulos]

I have committed your patch. Thanks again.

llvm/docs/TableGen/BackGuide.rst

 
 Each class or record built by TableGen is represented by an instance of
 the ``Record`` class. The ``RecordKeeper`` instance contains one map for the
 classes and one for the records. The primary data members of a record are
 the record name, the vector of field names and their values, and the vector of
 superclasses of the record.
 
 The record name is stored as a pointer to an ``Init`` (see `Init`_), which
-is a class whose instances hold TableGen values (sometimes refered to as
+is a class whose instances hold TableGen values (sometimes referred to as
 *initializers*). The field names and values are stored in a vector of
 ``RecordVal`` instances (see `RecordVal`_), each of which contains both the
 field name and its value. The superclass vector contains a sequence of
 pairs, with each pair including the superclass record and its source
 file location.
 
 In addition to those members, a ``Record`` instance contains:
 

 
 ``CodeInit``
 ~~~~~~~~~~~~
 
 The ``CodeInit`` class is a subclass of ``TypedInit``. Its instances
 represent arbitrary-length strings produced from ``code`` literals in the
 TableGen files. It includes a data member that contains a ``StringRef`` of
 the value. It also includes a data member specifying the source code
-location of the code stirng.
+location of the code string.
 
 The class provides the usual ``get()`` and ``getValue()`` functions. The
 latter function returns the ``StringRef``.
 
 The ``getLoc()`` function returns the source code location.
 
 
 ``DagInit``