set tab length to 2

Upstream uses indents of 4 spaces by default [1], but gitiles uses
2 spaces [2].  Change md_browser to match.

[1]: Python-Markdown/markdown#451
[2]: https://gerrit.googlesource.com/gitiles/+/master/Documentation/markdown.md#Lists

R=dpranke@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2772173002
Cr-Commit-Position: refs/heads/master@{#461334}

tools/md_browser/md_browser.py

@@ -165,6 +165,7 @@ def _DoMD(self, path):
 
     md = markdown.Markdown(extensions=extensions,
                            extension_configs=extension_configs,
+                           tab_length=2,
                            output_format='html4')
 
     has_a_single_h1 = (len([line for line in contents.splitlines()