Justification of text, by definition, will give some odd spacing. Word will not insert spaces within words; it expands/contracts the space between words. A URL counts as a word for this purpose, even if it contains spaces or underscores.
Alignment / Justification of Text in Microsoft Word

Personally, I would left-justify this.

Although I would not recommend it, you may want to try distributed text.
(Ctrl+Shift+J).