Request getParam only for Cake 3.4+

[TheoRenouf]

Hi,
On the page https://book.cakephp.org/3.0/en/controllers/request-response.html#request-parameters we can see an example explaining how to get parameters of the request. But that example works only for Cake 3.4+.

We should add an explication for Cake 3.x <3.4 users.

[ravage84]

https://api.cakephp.org/3.4/class-Cake.Http.ServerRequest.html#_param

[markstory]

Docs updated now.