[Docs][llvm-remarkutil] Fix dangling reference in documentation

D147710 introduced a new annotation-count subcommand to llvm-remarkutil
and added in documentation. However, the reference added under the
subcommands list never actually pointed to anything. This patch adds a
marker for the reference to point to so that the link works and the
sphinx build finishes without any errors.

llvm/docs/CommandGuide/llvm-remarkutil.rst

@@ -74,6 +74,8 @@ CSV format is as follows:
   Function,InstructionCount
   foo,123
 
+.. _annotation-count_subcommand:
+
 annotation-count
 ~~~~~~~~~~~~~~~~~