fix misuses of a vs an

Signed-off-by: cui fliter <imcusg@gmail.com>

echo.go

@@ -614,7 +614,7 @@ func (e *Echo) URL(h HandlerFunc, params ...interface{}) string {
 	return e.URI(h, params...)
 }
 
-// Reverse generates an URL from route name and provided parameters.
+// Reverse generates a URL from route name and provided parameters.
 func (e *Echo) Reverse(name string, params ...interface{}) string {
 	return e.router.Reverse(name, params...)
 }

router.go

@@ -151,7 +151,7 @@ func (r *Router) Routes() []*Route {
 	return routes
 }
 
-// Reverse generates an URL from route name and provided parameters.
+// Reverse generates a URL from route name and provided parameters.
 func (r *Router) Reverse(name string, params ...interface{}) string {
 	uri := new(bytes.Buffer)
 	ln := len(params)