fix RST reference in Writing an LLVM Pass

Currently, instead of showing up as link, it is rendered as

  ...of FunctionPass <writing-an-llvm-pass-FunctionPass>. The...

PR17733. Patch by Tay Ray Chuan!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193698 91177308-0d34-0410-b5e6-96231b3b80d8