replace LSTM with charRNN in examples

[shiffman]

The "reference" page I believe is correct but the example pages need the new charRNN naming rather than LSTM.

[hobg0blin]

put in a PR for this https://github.com/ml5js/ml5-website/pull/99

[seldomU]

The page https://ml5js.org/docs/CharRNN contains a link to an example. The link URL contains LSTM, it should be https://github.com/ml5js/ml5-examples/blob/master/p5js/CharRNN/CharRNN_Text/sketch.js