Fix generation speed calculation. (#2932)

Summary:
Pull Request resolved: #2932
overriding_review_checks_triggers_an_audit_and_retroactive_review
Oncall Short Name: executorch

Differential Revision: D55904722

fbshipit-source-id: 6057bc75f812e5ae9dd057bbed7291a539d80ff6
(cherry picked from commit 8cabeac)

Author: shoumikhin

examples/demo-apps/apple_ios/LLaMA/LLaMA/Application/ContentView.swift

@@ -215,7 +215,7 @@ struct ContentView: View {
           tokens.append(token)
           if tokens.count > 2 {
             let text = tokens.joined()
-            let count = text.count
+            let count = tokens.count
             tokens = []
             DispatchQueue.main.async {
               withAnimation {