Fix a button_to documentation example

[glacials]

Summary

Fixes an example in button_to's documentation; the description states button_to only takes a symbol for the method argument:

:method - Symbol of HTTP verb. Supported verbs are :post, :get, :delete, :patch, and :put. By default it will be :post.

but an example just below uses a string:

button_to('Destroy', 'http://www.example.com',
          method: "delete", remote: true, data: { confirm: 'Are you sure?', disable_with: 'loading...' }) %>

Passing an all-lowercase string incidentally works, so the example is technically functional despite being undefined behavior. But passing an all-uppercase string fails, and this is a pretty common practice when specifying HTTP methods (e.g. DELETE), so I think it's best this example is put in line with the documentation and converted to use a symbol.

Reproduction steps:

rails new buttonto
cd buttonto
rails c
# Prevent an error from calling button_to in the console
> def protect_against_forgery?
>   false
> end
=> :protect_against_forgery?

# Example verbatim (works)
>  button_to('Destroy', 'http://www.example.com', method: "delete", remote: true, data: { confirm: 'Are you sure?', disable_with: 'loading...' })
=> "<form class=\"button_to\" method=\"post\" action=\"http://www.example.com\" data-remote=\"true\"><input type=\"hidden\" name=\"_method\" value=\"delete\" /><input data-confirm=\"Are you sure?\" data-disable-with=\"loading...\" type=\"submit\" value=\"Destroy\" /></form>"

# Example with uppercase method (fails; renders a form without _method=delete)
>  button_to('Destroy', 'http://www.example.com', method: "DELETE", remote: true, data: { confirm: 'Are you sure?', disable_with: 'loading...' })
=> "<form class=\"button_to\" method=\"post\" action=\"http://www.example.com\" data-remote=\"true\"><input data-confirm=\"Are you sure?\" data-disable-with=\"loading...\" type=\"submit\" value=\"Destroy\" /></form>"

Other Information

I fell for this example myself and tried to pass an all-uppercase string after seeing the example without reading the options descriptions.

[gmcgibbon]

Thanks! For next time, please use [ci skip] for doc only changes.