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.

llvm-svn: 326999
diff --git a/llvm/docs/CommandGuide/index.rst b/llvm/docs/CommandGuide/index.rst
index 805df00..d596667 100644
--- a/llvm/docs/CommandGuide/index.rst
+++ b/llvm/docs/CommandGuide/index.rst
@@ -31,6 +31,7 @@
    llvm-symbolizer
    llvm-dwarfdump
    dsymutil
+   llvm-mca
 
 Debugging Tools
 ~~~~~~~~~~~~~~~