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(): Too few arguments in
vendor/behat/behat/src/Behat/Behat/Definition/DefinitionSnippet.php line 88
```

and 
``` gherkin
Given we are the 1%
```
will throw
```
[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.