Fix error on the pipeline-manipulation.md's sample code

    amp& was inserted in the g_clear_error(). It caused a compile error,
    so I removed it for making complie success.

Part-of: <gstreamer/gst-docs!85>

markdown/application-development/advanced/pipeline-manipulation.md

@@ -1288,7 +1288,7 @@ main (int argc, char **argv)
   g_option_context_add_group (ctx, gst_init_get_option_group ());
   if (!g_option_context_parse (ctx, &argc, &argv, &err)) {
     g_print ("Error initializing: %s\n", err->message);
-    g_clear_error (&err);
+    g_clear_error (&err);
     g_option_context_free (ctx);
     return 1;
   }