Update main.rs

[ReVoLt112]

Fix get plain text instead of HTML on 404

[gavtroy]

This doesn't seem like it would work. You're changing it for /raw/ which really should be Plain anyway, and the missing paste case for it is handled a few lines above. For reference on my branch I have this change to cover the case of 404s - https://github.com/gavtroy/pastebin/commit/5dae93998b9cf758d30cf23cf629863df8b3f5a1