rawvideoparse: fix typo in 'plane-offsets' description

The property is 'plane-offsets', not 'plane-offset' so the example in
the description was wrong.

https://bugzilla.gnome.org/show_bug.cgi?id=796817
diff --git a/gst/rawparse/gstrawvideoparse.c b/gst/rawparse/gstrawvideoparse.c
index 776e7fd..b1e955d 100644
--- a/gst/rawparse/gstrawvideoparse.c
+++ b/gst/rawparse/gstrawvideoparse.c
@@ -298,7 +298,7 @@
       PROP_PLANE_OFFSETS,
       gst_param_spec_array ("plane-offsets",
           "Plane offsets",
-          "Offsets of the planes in bytes (e.g. plane-offset=\"<0,76800>\")",
+          "Offsets of the planes in bytes (e.g. plane-offsets=\"<0,76800>\")",
           g_param_spec_int ("plane-offset",
               "Plane offset",
               "Offset of the n-th plane in bytes",