Fix typo

src/Terminal.ts

@@ -587,7 +587,7 @@ export class Terminal extends EventEmitter implements ITerminal, IDisposable, II
     }, true);
 
     on(this.element, 'keyup', (ev: KeyboardEvent) => {
-      if (!wasMondifierKeyOnlyEvent(ev)) {
+      if (!wasModifierKeyOnlyEvent(ev)) {
         this.focus();
       }
     }, true);
@@ -1929,7 +1929,7 @@ function off(el: any, type: string, handler: (event: Event) => any, capture: boo
   el.removeEventListener(type, handler, capture);
 }
 
-function wasMondifierKeyOnlyEvent(ev: KeyboardEvent): boolean {
+function wasModifierKeyOnlyEvent(ev: KeyboardEvent): boolean {
   return ev.keyCode === 16 || // Shift
     ev.keyCode === 17 || // Ctrl
     ev.keyCode === 18; // Alt