Fix example

[glaucocustodio]

The class method decode does not exist for Base64.

[grape-bot]

	1 Warning
⚠️	Unless you’re refactoring existing code, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#1780](https://github.com/ruby-grape/grape/pull/1780): Fix example - [@glaucocustodio](https://github.com/glaucocustodio).

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[dblock]

This is a good opportunity to ask the question whether we think calling method is OK here. It causes the service not to start because the method doesn't exist, maybe it should always be a lambda?

[glaucocustodio]

I think it is good to support method because give devs more possibilities. It seems good to me as well the service not to start because the method does not exist, it is easier to point errors.

[dblock]

Merged