Add missing space in help text for --print-dtd

[loic-sharma]

Improves the help text for --print-dtd. Before:

$ dart run -h -v
...
 --[no-]print-dtd                                      Prints connection details for the Dart Tooling
                                                       Daemon (DTD).Useful for Dart DevTools extension
                                                       authors working with DTD in the extension
                                                       development environment
...

I did not update the tests in pkg\dartdev\test\commands\run_test.dart as they don't seem to cover this help text.

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