[AMDGPU][MC][GFX9][NFC][DOC] Updated AMD GPU assembler syntax description.

Fixed bugs 48639, 49447, 49448, 49449.

GitOrigin-RevId: 424fe903d4d4b0f52bd4f86f62587efa6561d251
diff --git a/docs/AMDGPU/gfx9_opt.rst b/docs/AMDGPU/gfx9_opt.rst
index 19eefec..913bb51 100644
--- a/docs/AMDGPU/gfx9_opt.rst
+++ b/docs/AMDGPU/gfx9_opt.rst
@@ -5,9 +5,9 @@
     *                                                *
     **************************************************
 
-.. _amdgpu_synid9_opt:
+.. _amdgpu_synid_gfx9_opt:
 
 opt
-===========================
+===
 
 This is an optional operand. It must be used if and only if :ref:`glc<amdgpu_synid_glc>` is specified.