Fix copy/paste error in URLs for 'Roboto Mono' fonts
When switching to fetching fonts directly from Gerrit in
d46a08bd1223d84a6ff7819358173b18827db7d5, 'Roboto Mono' got pointed to
'Roboto' (without 'Mono') instead of 'Roboto Mono' (with 'Mono')
We point 'Roboto Mono' to the expected 'Mono' variants of 'Roboto'.