Add llvm-mca.rst to the table of contents in docs/CommandGuide.

This should fix the documentation error reported by builder llvm-sphinx-docs
(build #16407) after r326998.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326999 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed