DOC: typo on sigmoid function description

[guin0x]

Issue with current documentation:

In the sigmoid function it should be (1 / (1 + exp(-x)) instead of (1 / (1 + exp(x)). The relevant line is this.

Idea or request for content:

No response

[twiecki]

@guin0x Want to do a PR?

[aseyboldt]

Fixed in #157, thanks again @guin0x