[docs][GlobalISel] Fix the warnings during docs-llvm-html build

Before the fix the build of docs-llvm-html would fail.
The rG8bc03d216824 introduced a reference to an undefined label,
so we have warning as:

  llvm-project/llvm/docs/GlobalISel/GenericOpcode.rst:295:\
  undefined label: i_intr_llvm_ptrmask (if the link has no\
  caption the label must precede a section header)