Use the system default version of Python to run Documentation scripts.

They work fine with Python 2.7 and Python 3.5 anyway and some systems do not
have a "python2" binary (e.g. macOS). This will fix the Gerrit CI build on
Bazel's CI, which is currently failing with:

ERROR: /Users/buildkite/builds/buildkite-macpro-7-1/bazel-downstream-projects/gerrit/Documentation/BUILD:94:1: Couldn't build file Documentation/pg-plugin-endpoints.txt.html_htmlonly_expn: error executing shell command: '.html_htmlonly_expn' failed (Exit 127)
env: python2: No such file or directory

I tested this change like this:

# Build with Python 2 to ensure my local setup is fine.
bazel build //Documentation/...
# Works fine.

# Now build with Python 3 instead, to ensure compatibility.
sed -i 's/python2/python3/g' Documentation/replace_macros.py \
    tools/bzl/license.bzl
bazel clean
bazel build //Documentation/...
# Works fine, too.

Thus we can safely change "python2" to "python" and let the scripts run with
the system default version.

Change-Id: I107b76bc71e9b42101d57f94ab7c54c90e7c74cb
diff --git a/Documentation/replace_macros.py b/Documentation/replace_macros.py
index 09e2439..c76d133 100755
--- a/Documentation/replace_macros.py
+++ b/Documentation/replace_macros.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python
 # coding=utf-8
 # Copyright (C) 2013 The Android Open Source Project
 #