fix shebang line in html5check.py to run on any Unix and Linux system

I propose this change, which makes script work on any Unix / Linux platform. Fix tested on OpenBSD.

```
diff --git a/html5check.py b/html5check.py
index 703a35f..9fb1e72 100755
--- a/html5check.py
+++ b/html5check.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#! /usr/bin/env python
```