fix typo

switched of -> switched off

Author: hankache

doc/Language/variables.pod6

@@ -882,7 +882,7 @@ re-applied by assigning C<Nil> to it:
 =head2 Default Defined Variables Pragma
 
 To force all variables to have a definedness constraint use the pragma C<use
-variables :D>. The pragma is lexically scoped and can be switched of with C<use
+variables :D>. The pragma is lexically scoped and can be switched off with C<use
 variables :_>.
 
 =begin code :skip-test