Clarify the Euler's Number constant in the sigmoid function example

[jochasinga]

It is better to be explicit with the Euler's Number in the equation and add a short explanation and a way to use the constant in Julia than to use expr which is inconsistent with the code that followed.

Also correct 'e' to unicode e in the code followed (consider using expr here if it makes the code clearer).

Minor: format g function name as code.

[logankilpatrick]

Thanks for this!