Minor RNNCell docs fix (#62)

* Minor RNNCell docs fix

Matches the preceding example

* rnn -> rnn2

docs/src/models/recurrence.md

@@ -50,12 +50,12 @@ We sometimes refer to functions like `rnn` above, which explicitly manage state,
 ```julia
 using Flux
 
-m = Flux.RNNCell(10, 5)
+rnn2 = Flux.RNNCell(10, 5)
 
 x = rand(10) # dummy data
 h = rand(5)  # initial hidden state
 
-h, y = rnn(h, x)
+h, y = rnn2(h, x)
 ```
 
 ## Stateful Models