syntax: nanohelp: colorize also ^/ as a possible keystroke

(In addition, colorize F20...F24 too.)

This fixes https://savannah.gnu.org/bugs/?57390.

Bug existed since version 4.6, commit 2a97cb9f.

syntax/nanohelp.nanorc

@@ -4,7 +4,7 @@
 syntax nanohelp
 
 # Key combos:
-color cyan "\^[]4-8A-Z^\_◀▶▲▼]"  "[◀▶▲▼]"  "\<M-."  "\<F1?[0-9]"
+color cyan "\^[]/4-8A-Z\^_◀▶▲▼]"  "[◀▶▲▼]"  "\<M-."  "\<F([1-9]|1[0-9]|2[0-4])"
 color cyan "\<((Sh-)?Tab|Enter|Ins|(Sh-\^?)?Del|Space|Bsp|Up|Down|Left|Right|Home|End|PgUp|PgDn)\>"
 
 # Quoted indicators: