Fix snippet generation when step contains percentage symbol

Otherwise the following undefined step 

``` gherkin
Given 1% of scenarios' steps contains a percentage symbol
```

will throw the following exception:

```
[ErrorException]

Warning: sprintf(): Argument number must be greater than zero in
/vendor/behat/behat/src/Behat/Behat/Definition/DefinitionSnippet.php line 87
```

because the `%` symbol isn't escaped.