docs(stepfunctions-tasks): EvaluateExpression only supports NODEJS_10_X (#13227)

### Description
This PR documents only NODEJS_10_X for EvaluateExpression. It was mistakenly changed to NODEJS_12_X in bulk PR #12807

Issue which requests supports NODEJS_12_X version by default in EvaluateExpression #13224

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: trivikr

packages/@aws-cdk/aws-stepfunctions-tasks/README.md

@@ -188,8 +188,8 @@ const convertToSeconds = new tasks.EvaluateExpression(this, 'Convert to seconds'
 
 const createMessage = new tasks.EvaluateExpression(this, 'Create message', {
   // Note: this is a string inside a string.
-    expression: '`Now waiting ${$.waitSeconds} seconds...`',
-    runtime: lambda.Runtime.NODEJS_12_X,
+  expression: '`Now waiting ${$.waitSeconds} seconds...`',
+  runtime: lambda.Runtime.NODEJS_10_X,
   resultPath: '$.message',
 });
 
@@ -213,7 +213,7 @@ new sfn.StateMachine(this, 'StateMachine', {
 
 The `EvaluateExpression` supports a `runtime` prop to specify the Lambda
 runtime to use to evaluate the expression. Currently, the only runtime
-supported is `lambda.Runtime.NODEJS_12_X`.
+supported is `lambda.Runtime.NODEJS_10_X`.
 
 
 ## Athena