docs: for the alternative bindings, rebind ^C only in the main menu

Because ^C is needed for Cancel in most other menus.

This fixes https://bugs.debian.org/950223.
Reported-by: Jonas Smedegaard <dr@jones.dk>

Bug existed since version 4.0, commit 3a170d2c.

doc/sample.nanorc.in

@@ -281,7 +281,7 @@
 #unbind ^Y all
 #bind M-X flipnewbuffer all
 #bind ^X cut all
-#bind ^C copy all
+#bind ^C copy main
 #bind ^V paste all
 #bind ^P curpos main
 #bind ^T gotoline main