docs: fix escape sequences in utils functions
docs: fix escape sequences in utils functions

gtk-doc treats backslash as the escape character, so devhelp displays
"\." as "." and "\\" as "\". In the documentation for
gtk_source_utils_escape_search_text() and
gtk_source_utils_unescape_search_text(), we want to display "\" and "\\"
literally so they have to be surrounded by backticks.

https://bugzilla.gnome.org/show_bug.cgi?id=768040

GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,uajain/gtksourceview