Documentation erroneously states that `coef_[0]` is the intercept (bias)

[Florent-H]

In the pygam.pygam.LinearGAM section on readthedocs, the documentation states that coef_[0] is the intercept (bias), while in version 0.9.0, it is in fact coef_[-1].

It would be a quick fix that would prevent new users like me from making a mistake in usage.

[dswah]

thanks for pointing this out!