Fix invalid regular expression

[jeppert]

Updated regular expression matching invalid domain names

Regular expression used \w in the part the matches the domain. A word (\w) matches with '_' which is an invalid character in a domain name.

Summary

The second part of the regular expression matched for underscore, an invalid character in a domain name. Updated the regular expression and the descriptive text to simply match for alpha numeric characters.

[dnfclas]

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. We will now review your pull request.
Thanks,
.NET Foundation Pull Request Bot

[rpetrusha]

Thanks for contributing to the dotnet/docs repo and improving this regex, @jeppert. We'll merge your PR now. Changes should appear on docs.microsoft.com in the next day or two.