stack: Fix typo in comments

[Bruno Haible]

A trivial typo.


2020-12-26  Bruno Haible  <bruno@clisp.org>

        stack: Fix typo in comments.
        * lib/stack.h: Fix typo in comments.

diff --git a/lib/stack.h b/lib/stack.h
index 5d80390..7f19d26 100644
--- a/lib/stack.h
+++ b/lib/stack.h
@@ -53,8 +53,8 @@
    After including this file, these names will be undefined.
 
    Before including this file, you also need to include:
-     #include "<stdbool.h>"
-     #include "<stdlib.h>"
+     #include <stdbool.h>
+     #include <stdlib.h>
      #include "assure.h"
      #include "xalloc.h"
 */