added example of ignoring a specific warning

[MartinoMensio]

Description

This PR includes an example for ignoring a specific warning that needs to be done differently from version 2.2 (breaking change). Since I never used the warnings package before and probably many spaCy users too, I think that an example is needed.
I also hope that this is the most appropriate way of ignoring one of the warnings declared and used by spaCy.

Types of change

It is a change in the documentation, adding a code example

Checklist

• I have submitted the spaCy Contributor Agreement.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[adrianeboyd]

Thanks, this is a helpful addition! I'd like to modify it slightly, but don't want to edit it here since it would push to your master branch (in general it's good idea to use a branch for PRs), so I'll create a new PR instead.