DOC: Update flake8 command instructions
xref <a href="https://github.com/pandas-dev/pandas/pull/16738#issuecomment-315211329">#16738 (comment)</a>

cc @thequackdaddy 

Ignore the Appveyor failure.  That's it being silly on a PR with `[ci skip]`. @jreback : Just curious, do I have similar permissions as I do on Travis for builds?  I tried navigating to one and didn't see any options to restart builds or anything. @gfyoung login using pandas-dev username; it will validate using github and you should be able to interact > login using pandas-dev username

If I try clicking the `GitHub` button, it logs in using my `GitHub` account (and can't interact with builds).  However, if I try entering `pandas-dev` into the username-password field, I need a password 😉 - what am I doing wrong here? @thequackdaddy : if you have anything you would like to add, let me know.  Otherwise, I'll merge this later today or tomorrow (without the `ci skip`).