[docs] Fixup fallout from other grammar fixup.

My "excuse" for not refactoring the grammar here is to not diverge too
far from the grammar in the comments of TGParser.cpp, since I'm not
taking on the quest of majorly refactoring TGParser.cpp at the moment.

One benefit of doing this is that Ideas for refactoring and clarifying
the grammar in this document should translate almost immediately to
beneficial refactorings that can be made to TGParser.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174144 91177308-0d34-0410-b5e6-96231b3b80d8