videotestsrc: fix build if adder is disabled

The videotestsrc is not build if the adder plugin is disabled. This is a
copy/paste error introduced in Commit 3de86b2b9725 ("docs: port plugins to
explicit sources").

Fix the check to actually test the 'videotestsrc' option.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8399>

subprojects/gst-plugins-base/gst/videotestsrc/meson.build

@@ -17,7 +17,7 @@ plugin_sources += {
   'videotestsrc': pathsep.join(doc_sources)
 }
 
-if get_option('adder').disabled()
+if get_option('videotestsrc').disabled()
   subdir_done()
 endif