DOC: Wrong URL in web/README.md
#### Location of the documentation

**Note**: You can check the latest versions of the docs on `master` [here](https://pandas.pydata.org/docs/dev/).

#### Documentation problem

`web/README.md` states
`Directory containing the pandas website (hosted at https://pandas.io).
`

However, [pandas.io](https://pandas.io) is not reachable.

#### Suggested fix for documentation

We can replace it with the following:

`Directory containing the pandas website (hosted at https://pandas.pydata.org/).
`