Fix comments of GUC hooks for timezone_abbreviations

The GUC assign and check hooks used "assign_timezone_abbreviations",
which was incorrect.

Issue noticed while browsing this area of the code, introduced in
0a20ff54f5.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/Z1eV6Y8yk77GZhZI@paquier.xyz
Backpatch-through: 16

src/backend/commands/variable.c

@@ -479,7 +479,7 @@ show_log_timezone(void)
  */
 
 /*
- * GUC check_hook for assign_timezone_abbreviations
+ * GUC check_hook for timezone_abbreviations
  */
 bool
 check_timezone_abbreviations(char **newval, void **extra, GucSource source)
@@ -511,7 +511,7 @@ check_timezone_abbreviations(char **newval, void **extra, GucSource source)
 }
 
 /*
- * GUC assign_hook for assign_timezone_abbreviations
+ * GUC assign_hook for timezone_abbreviations
  */
 void
 assign_timezone_abbreviations(const char *newval, void *extra)