alsamidisrc: Improve a comment and remove one trailing space

https://bugzilla.gnome.org/show_bug.cgi?id=787683

ext/alsa/gstalsamidisrc.c

@@ -122,7 +122,7 @@ parse_ports (const char *arg, GstAlsaMidiSrc * alsamidisrc)
   /*
    * Assume that ports are separated by commas.
    *
-   * Commas are used instead of spaces because those are valid in client
+   * Commas are used instead of spaces because spaces are valid in client
    * names.
    */
   ports_list = g_strsplit (arg, ",", 0);