build, doc: Adjust comment after PR27254

Author: hebasto

configure.ac

@@ -1075,7 +1075,7 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <malloc.h>]],
 dnl Check for posix_fallocate
 AC_MSG_CHECKING([for posix_fallocate])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
-                   // same as in src/util/system.cpp
+                   // same as in src/util/fs_helpers.cpp
                    #ifdef __linux__
                    #ifdef _POSIX_C_SOURCE
                    #undef _POSIX_C_SOURCE