moves php:namespace after page title in validation.rst

I noticed that the validation page has no bold title on the search-result popup:

![screen shot 2014-12-10 at 18 54 50](https://cloud.githubusercontent.com/assets/143224/5381077/3cdf7e7e-809e-11e4-8bf8-7cb5495e24e5.png)

I was lazy and didn't check the code, but I guess the reason is that the title tag is not first on page?