docs: Remove '\' escape character from literals

commit 7f6a964c47 replaced entities, but
escaped the replacement text also inside literals, which resulted in the
escaping '\' to also appear in the documentation.

https://bugzilla.gnome.org/show_bug.cgi?id=727322

gtk/gtkshow.c

@@ -39,7 +39,7 @@
  * Typical examples are
  * - `file:///home/gnome/pict.jpg`
  * - `http://www.gnome.org`
- * - `mailto:me\@gnome.org`
+ * - `mailto:me@gnome.org`
  * Ideally the timestamp is taken from the event triggering
  * the gtk_show_uri() call. If timestamp is not known you can take
  * %GDK_CURRENT_TIME.