key_generic_help(): Use display_help()

Now that we moved the online help code into a separate function, use
that to display the introduction help page when pressing the generic
help key binding.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.c

@@ -278,7 +278,7 @@ static inline void key_view_note(void)
 
 static inline void key_generic_help(void)
 {
-	wins_launch_external(DOCDIR "/intro.txt", conf.pager);
+	display_help(NULL);
 	wins_update(FLAG_ALL);
 }