Help page not updated in 3.4.0

[0tkl]

https://github.com/TypesettingTools/Aegisub/blob/b0fc741099f610ee9486bfaf0186d39f74b8d756/src/help_button.cpp#L80-L87

Once aegisite provides a dedicated /3.4/ route (instead of the /latest/ route at the moment), the link in Aegisub could be updated consequently.