Update routing.md <PARAM>
Update routing.md <PARAM>

Some markdown readers (such as github's preview) parse `<PARAM>` as an HTML tag.

Wrapping it in a code block fixes that.
r0fls/sanic,ai0/sanic,yunstanford/sanic,r0fls/sanic,channelcat/sanic,yunstanford/sanic,jrocketfingers/sanic,ashleysommer/sanic,lixxu/sanic,Tim-Erwin/sanic,channelcat/sanic,Tim-Erwin/sanic,ai0/sanic,yunstanford/sanic,jrocketfingers/sanic,ashleysommer/sanic,lixxu/sanic,yunstanford/sanic,ashleysommer/sanic,channelcat/sanic,lixxu/sanic,channelcat/sanic,lixxu/sanic