Fix namespace for MethodNotFoundException

Currently, I get:

````PHP
PHP Fatal error: Class 'Prophecy\Exception\Prophecy\MethodNotFoundException' not found in /[...]/vendor/phpspec/prophecy/src/Prophecy/Doubler/Generator/ClassCreator.php(48) : eval()'d code on line 22
`````

when calling a non-existing method on a revealed object.

I believe this exception should be `\Prophecy\Exception\Doubler\MethodNotFoundException`.