BLD: Remove csv dialect warning in io.rst
Title is self-explanatory.

xref <a href="https://github.com/pandas-dev/pandas/pull/14911#issuecomment-273896473">#14911 (comment)</a>.
maybe I wasn't clear. Is this a legit warning?  ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15177?src=pr) is 85.54% (diff: 100%)
> Merging [#15177](https://codecov.io/gh/pandas-dev/pandas/pull/15177?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

```diff
@@             master     #15177   diff @@
==========================================
  Files           145        145          
  Lines         51406      51406          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43978      43977     -1   
- Misses         7428       7429     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [d50eaa8...1ac6c20](https://codecov.io/gh/pandas-dev/pandas/compare/d50eaa8ac6e18a64d9fa403e8e263344a5208227...1ac6c20b08c28c6db64a83a6d550747be90fd7c0?src=pr) IMO it should not warn. But that was what I advocated for in the PR #14911, and I lost that argument ..

So @jreback "is this a legit warning?", yes, given how we merged the PR, this warning is expected and not really avoidable (just annoying and IMO unnecessary). So using `:okwarning:` is the right thing to do then. ok then thanks!