Fix wrong output message when invoking with --coverage-text switch

When invoking `phpunit --coverage-text`, following output is shown:

    ...
    Generating code coverage report in HTML format ... done
    ...

This commit modifies message to:

    ...
    Generating code coverage report in Text format ... done
    ...

Fix could be optimized, but since I haven't found an official coding standards, I went with a safe approach :)