[Doc][NFC] Add template type when use MachinePassRegistry.

[Jimerlife]

Summary

Update WritingAnLLVMPass.rst, add template type when use MachinePassRegistry.

llvm/docs/WritingAnLLVMPass.rst

 recommend ``llvm/CodeGen/RegAllocRegistry.h``.  The key things to modify are
 the class name and the ``FunctionPassCtor`` type.
 
 Then you need to declare the registry.  Example: if your pass registry is
 ``RegisterMyPasses`` then define:
 
 .. code-block:: c++
 
-  MachinePassRegistry RegisterMyPasses::Registry;
+  MachinePassRegistry<RegisterMyPasses::FunctionPassCtor> RegisterMyPasses::Registry;
 
 And finally, declare the command line option for your passes.  Example:
 
 .. code-block:: c++
 
   cl::opt<RegisterMyPasses::FunctionPassCtor, false,
           RegisterPassParser<RegisterMyPasses> >
   MyPassOpt("mypass",