[docs] Fix typo in the example code in ProgrammersManual.

[dexonsmith]

Thanks for finding this! I think processBuffer() pairs better with processBufferAtPath() though so I suggest a different fix...

LGTM if you go with my alternate suggestion.

llvm/docs/ProgrammersManual.rst

579

I think this should be left alone...

588

... and this should be changed to processBuffer().

[HsiangKai]

Address @dexonsmith's comments.

llvm/docs/ProgrammersManual.rst

 If an ``Expected<T>`` value will be moved into an existing variable then the
 ``moveInto()`` method avoids the need to name an extra variable.  This is
 useful to enable ``operator->()`` the ``Expected<T>`` value has pointer-like
 semantics.  For example:
 
 .. code-block:: c++
 
   Expected<std::unique_ptr<MemoryBuffer>> openBuffer(StringRef Path);
   Error processBuffer(StringRef Buffer);
 
   Error processBufferAtPath(StringRef Path) {
     // Try to open a buffer.
     std::unique_ptr<MemoryBuffer> MB;
     if (auto Err = openBuffer(Path).moveInto(MB))
       // On error, return the Error value.
       return Err;
     // On success, use MB.
-    return processContent(MB->getBuffer());
+    return processBuffer(MB->getBuffer());
   }
 
 This third form works with any type that can be assigned to from ``T&&``. This
 can be useful if the ``Expected<T>`` value needs to be stored an already-declared
 ``Optional<T>``. For example:
 
 .. code-block:: c++