var MatchedRoutePathParam = "$matchedRoutePath" changed declaration to const MatchedRoutePathParam

[LeoDiKadyrov]

I only suggested it due to following reasons:

• John Carmack once said: "I am a full const nazi nowadays, and I chide any programmer that doesn’t const every variable and parameter that can be."
• I'm a newbie into development, trying to learn staff and while studying source code of this library, I saw an opportunity for small change and my first PR into open source ever

All tests that are originally in master repo passed green. When I was going through the code it just confused me a bit, that a variable only used in router.go and not really mutated anywhere declared as var. Sorry if it's too small to have anyone attention and a whole PR open for it.