Fix typo in router error message (#5515)

Tiny typo fix 🙂

packages/next-server/lib/EventEmitter.js

@@ -7,7 +7,7 @@ export default class EventEmitter {
     }
 
     if (this.listeners[event].has(cb)) {
-      throw new Error(`The listener already exising in event: ${event}`)
+      throw new Error(`Listener already exists for router event: \`${event}\``)
     }
 
     this.listeners[event].add(cb)

test/unit/EventEmitter.test.js

@@ -69,7 +69,7 @@ describe('EventEmitter', () => {
 
       const run = () => ev.on('sample', cb)
 
-      expect(run).toThrow(/The listener already exising in event: sample/)
+      expect(run).toThrow(/Listener already exists for router event: `sample`/)
     })
 
     it('should support chaining like the nodejs EventEmitter', () => {