docs: Fix a couple of typo-ish errors in WritingAnLLVMPass

- Make CallGraphSCCPass's paragraph about doFinalization refer to
  runOnSCC instead of runOnFunction, since that's what it's about.
- Fix a reference in the FunctionPass paragraph.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222222 91177308-0d34-0410-b5e6-96231b3b80d8