Cleanup: use single back-tick quoting in comments

While doxygen supports both, conform to our style guide.

Note that single back-tick's are already used in a majority of comments.

source/blender/python/gpu/gpu_py_shader.c

@@ -17,8 +17,8 @@
 /** \file
  * \ingroup bpygpu
  *
- * - Use ``bpygpu_`` for local API.
- * - Use ``BPyGPU`` for public API.
+ * - Use `bpygpu_` for local API.
+ * - Use `BPyGPU` for public API.
  */
 
 #include <Python.h>