doc: Fix doc comment for GstChildProxy
This removes warning like:
../subprojects/gstreamer/gst/gstchildproxy.h:57: Error: Gst: identifier not found on the first line:
* #GstChildProxyInterface::get_child_by_name:
^
This removes warning like:
../subprojects/gstreamer/gst/gstchildproxy.h:57: Error: Gst: identifier not found on the first line:
* #GstChildProxyInterface::get_child_by_name:
^