docs: mark input/output block locations as DONE

Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>

docs/GL3.txt

@@ -184,7 +184,7 @@ GL 4.4, GLSL 4.40:
   - forced alignment within blocks                     in progress
   - specified vec4-slot component numbers              in progress
   - specified transform/feedback layout                in progress
-  - input/output block locations                       in progress
+  - input/output block locations                       DONE
   GL_ARB_multi_bind                                    DONE (all drivers)
   GL_ARB_query_buffer_object                           not started
   GL_ARB_texture_mirror_clamp_to_edge                  DONE (i965, nv50, nvc0, r600, radeonsi, llvmpipe, softpipe)