Add period at end of sentence

Author: logankilpatrick

docs/src/models/advanced.md

@@ -30,7 +30,7 @@ model = CustomModel(chain)
 model(rand(10))
 ```
 
-For an intro to Flux and automatic differentiation, see this [tutorial](https://fluxml.ai/tutorials/2020/09/15/deep-learning-flux.html)
+For an intro to Flux and automatic differentiation, see this [tutorial](https://fluxml.ai/tutorials/2020/09/15/deep-learning-flux.html).
 
 ## Customising Parameter Collection for a Model