API Docs: Fix corrupted document of gpu.select.load_id

The Python API document of gpu.select.load_id does not follow rst format.
c.f. https://docs.blender.org/api/current/gpu.select.html

This patch fixes it.

Reviewed By: Blendify

Differential Revision: https://developer.blender.org/D8547

source/blender/python/gpu/gpu_py_select.c

@@ -41,7 +41,7 @@
  * \{ */
 
 PyDoc_STRVAR(bpygpu_select_load_id_doc,
-             "load_id(id)\n"
+             ".. function:: load_id(id)\n"
              "\n"
              "   Set the selection ID.\n"
              "\n"