Fix typo in GetOrAdd usage instructions

Author: bgavrilMS

.github/copilot-instructions.md

@@ -20,7 +20,7 @@ pool.GetOrAdd(key, _ => new ExpensiveObject());
 pool.GetOrAdd(key, _ => CreateExpensiveObject());
 ```
 
-When reviewing code, flag any `GetOrAdd` call where the second argument is **not** a delegate/lambda/method group. All other usages if GetOrAdd(key, value) should be justified.
+When reviewing code, flag any `GetOrAdd` call where the second argument is **not** a delegate/lambda/method group. All other usages of GetOrAdd(key, value) should be justified.
 
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