Fix an example in the callback documentation

Add return value to make the demo code more accurate

Bug: 1151318
Change-Id: Ia300589a5d0a722c9e3d4bbe3231ea69793df24b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2752668
Reviewed-by: danakj <[email protected]>
Commit-Queue: danakj <[email protected]>
Cr-Commit-Position: refs/heads/master@{#862243}
diff --git a/docs/callback.md b/docs/callback.md
index 1b51645..c97c86d 100644
--- a/docs/callback.md
+++ b/docs/callback.md
@@ -608,7 +608,10 @@
 doesn't expect a return value.
 
 ```cpp
-int DoSomething(int arg) { cout << arg << endl; }
+int DoSomething(int arg) {
+  cout << arg << endl;
+  return arg;
+}
 base::RepeatingCallback<void(int)> cb =
     base::BindRepeating(IgnoreResult(&DoSomething));
 ```