doc: drm: Fix mis-spelling of i915_guc_submission includes

In commit
 d1675198: drm/i915: Integrate GuC-based command submission

the drm.tmpl include lines reference the intel_guc_submission.c but the
patch adds the file i915_guc_submission.c. drm.tmpl fails to build with:
 docproc: .//drivers/gpu/drm/i915/intel_guc_submission.c:
  No such file or directory

Change the file reference to the actual file.
Signed-off-by: Graham Whaley <graham.whaley@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Documentation/DocBook/drm.tmpl

@@ -4246,8 +4246,8 @@ int num_ioctls;</synopsis>
       </sect2>
       <sect2>
         <title>GuC Client</title>
-!Pdrivers/gpu/drm/i915/intel_guc_submission.c GuC-based command submissison
-!Idrivers/gpu/drm/i915/intel_guc_submission.c
+!Pdrivers/gpu/drm/i915/i915_guc_submission.c GuC-based command submissison
+!Idrivers/gpu/drm/i915/i915_guc_submission.c
       </sect2>
     </sect1>