Fix typo

Author: kocsismate

build/php.m4

@@ -1900,7 +1900,7 @@ AC_DEFUN([PHP_PROG_PHP],[
     set $php_version
     IFS=$ac_IFS
     php_version_num=`expr [$]{1:-0} \* 10000 + [$]{2:-0} \* 100 + [$]{3:-0}`
-    dnl Minimum supported version for gen_stubs.php is PHP 7.4.
+    dnl Minimum supported version for gen_stub.php is PHP 7.4.
     if test "$php_version_num" -lt 70400; then
       AC_MSG_RESULT([$php_version (too old)])
       unset PHP