not used at → used outside

* private.h (static_assert): Improve comment.

Author: eggert

private.h

@@ -83,7 +83,7 @@
 #endif
 
 /* For pre-C23 compilers, a substitute for static_assert.
-   Some of these compilers may warn if it is not used at the top level.  */
+   Some of these compilers may warn if it is used outside the top level.  */
 #if __STDC_VERSION__ < 202311 && !defined static_assert
 # define static_assert(cond) extern int static_assert_check[(cond) ? 1 : -1]
 #endif