correct FLEXPTR_* example in comment

This section is about "The FLEXPTR_* variants", so use FLEXPTR_ALLOC_STR
in the example.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -803,7 +803,7 @@ extern FILE *fopen_for_writing(const char *path);
  * you can do:
  *
  *   struct foo *f;
- *   FLEX_ALLOC_STR(f, name, src);
+ *   FLEXPTR_ALLOC_STR(f, name, src);
  *
  * and "name" will point to a block of memory after the struct, which will be
  * freed along with the struct (but the pointer can be repointed anywhere).