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

docs/WritingAnLLVMPass.rst

@@ -131,7 +131,7 @@ Next, we declare our pass itself:
 
   struct Hello : public FunctionPass {
 
-This declares a "``Hello``" class that is a subclass of `FunctionPass
+This declares a "``Hello``" class that is a subclass of :ref:`FunctionPass
 <writing-an-llvm-pass-FunctionPass>`.  The different builtin pass subclasses
 are described in detail :ref:`later <writing-an-llvm-pass-pass-classes>`, but
 for now, know that ``FunctionPass`` operates on a function at a time.