Fix obvious typo

The -fcf-protection option takes an optional argument, which allows the
requesting of control-flow protection on returns, or on indirect jumps
and calls, or both. One of the comments in the code applies to returns,
yet it mentions branches (for indirect calls and jumps). This patch
fixes this obvious typo.

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D117836