execute: Update comment

[Bruno Haible]

pipe.h was renamed to spawn-pipe.h in 2010.


2018-04-07  Bruno Haible  <address@hidden>

        execute: Update comment.
        * lib/execute.h (execute): Refer to spawn-pipe.h, not pipe.h.

diff --git a/lib/execute.h b/lib/execute.h
index dfa39a3..f8f92b0 100644
--- a/lib/execute.h
+++ b/lib/execute.h
@@ -33,7 +33,7 @@
    the subprocess (if supported by the platform: not on native Windows
    platforms), otherwise 0.
    It is recommended that no signal is blocked or ignored while execute()
-   is called.  See pipe.h for the reason.  */
+   is called.  See spawn-pipe.h for the reason.  */
 extern int execute (const char *progname,
                     const char *prog_path, char **prog_argv,
                     bool ignore_sigpipe,