Fix shebang line for //testing/libfuzzer/coverage.py.

The file had "/usr/bin/python2", but that doesn't exist on the mac.
This changes the line to "/usr/bin/env python" which is what we
normally recommend; python == python2 on nearly every distribution.

R=mmoroz@chromium.org

Change-Id: I67f37864d8a5e80cbf79b5ff52b00041974d2ae9
Reviewed-on: https://chromium-review.googlesource.com/696137
Commit-Queue: Abhishek Arya <inferno@chromium.org>
Reviewed-by: Abhishek Arya <inferno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505893}

testing/libfuzzer/coverage.py

-#!/usr/bin/python2
+#!/usr/bin/env python
 #
 # Copyright 2017 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be