kconfig: correct minor typo in Kconfig warning message.

Correct a minor spelling mistake in a Kconfig warning message.
Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/kconfig/menu.c

@@ -203,7 +203,7 @@ void sym_check_prop(struct symbol *sym)
 			else if (sym2->type == S_UNKNOWN)
 				prop_warn(prop,
 				    "'select' used by config symbol '%s' "
-				    "refer to undefined symbol '%s'",
+				    "refers to undefined symbol '%s'",
 				    sym->name, sym2->name);
 			else if (sym2->type != S_BOOLEAN && sym2->type != S_TRISTATE)
 				prop_warn(prop,