DOC: fix numpy docstr example Vectorizer()

[sappelhoff]

I added these changes because the previous example was confusing in some ways, for example:

• what is SpatialFilter?
• what is _XDawnTransformer, and why do we use a private class here?
• the example did not render due to lack of >>>

How it was before this PR:

How it is now:

[larsoner]

Nice, thanks @sappelhoff !