[MRG] DOC A few fixes in the docstrings of all classifiers.

Make the `[n_classes, n_samples]` appear without any extra spaces in
between the two words.

I was getting the output of the docstring like this:

````
    Attributes
    ----------
    coef_ : array, shape = [1, n_features] if n_classes == 2 else [n_classes,            n_features]

```

A very minor PR, @jnothman could you take a look...
If you feel this change is neater, please let me know this needs to be documented under contributors guide.