Fixing the following warning when generating documentation:
xml/element-gaussianblur.xml:72: element refsect2: validity error :
    ID GstGaussianBlur already defined
<refsect2 id="GstGaussianBlur" role="typedef">
                                             ^
Warning: multiple "IDs" for constraint linkend: GstGaussianBlur.
  DOC   Fixing cross-references