Fix up doc formatting, use spaces for alignment (tabs are for indentation only).

This fixes issue where if user has tab width other than 4, some lines will become misaligned. For example, see the package description at https://godoc.org/github.com/gorilla/mux.

doc.go

@@ -89,7 +89,7 @@ There are several other matchers that can be added. To match path prefixes:
 
 	r.MatcherFunc(func(r *http.Request, rm *RouteMatch) bool {
 		return r.ProtoMajor == 0
-    })
+	})
 
 ...and finally, it is possible to combine several matchers in a single route:
 
@@ -164,8 +164,8 @@ This also works for host variables:
 
 	// url.String() will be "http://news.domain.com/articles/technology/42"
 	url, err := r.Get("article").URL("subdomain", "news",
-									 "category", "technology",
-									 "id", "42")
+	                                 "category", "technology",
+	                                 "id", "42")
 
 All variables defined in the route are required, and their values must
 conform to the corresponding patterns. These requirements guarantee that a
@@ -193,7 +193,7 @@ as well:
 
 	// "http://news.domain.com/articles/technology/42"
 	url, err := r.Get("article").URL("subdomain", "news",
-									 "category", "technology",
-									 "id", "42")
+	                                 "category", "technology",
+	                                 "id", "42")
 */
 package mux