drm/doc: Remove duplicate "by"

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Documentation/DocBook/drm.tmpl

@@ -1377,7 +1377,7 @@ int max_width, max_height;</synopsis>
       <itemizedlist>
         <listitem>
         DRM_PLANE_TYPE_PRIMARY represents a "main" plane for a CRTC.  Primary
-        planes are the planes operated upon by by CRTC modesetting and flipping
+        planes are the planes operated upon by CRTC modesetting and flipping
         operations described in <xref linkend="drm-kms-crtcops"/>.
         </listitem>
         <listitem>