docs(aws-cloudwatch): Fix example for MathExpression (#22933)

In the commit f37991c the example was changed to use a different metric to make it compile. Unfortunately the `id` for the metric was only updated in its use not in the definition.

This can be a bit confusing when reading the example for the first time and if used as is it would not result in the intended behavior. Therefore I updated the example.

----

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packages/@aws-cdk/aws-cloudwatch/README.md

@@ -75,7 +75,7 @@ const allProblems = new cloudwatch.MathExpression({
   expression: "errors + throttles",
   usingMetrics: {
     errors: fn.metricErrors(),
-    faults: fn.metricThrottles(),
+    throttles: fn.metricThrottles(),
   }
 });
 ```