Fix nthChild example (#609)

was incorrect, didn't match playground

Author: dwgrth

website/guide/rule-config/atomic-rule.md

@@ -177,7 +177,7 @@ The [following rule](/playground.html#eyJtb2RlIjoiQ29uZmlnIiwibGFuZyI6ImphdmFzY3
 ```yaml
 rule:
   kind: number
-  nthChild: 1
+  nthChild: 2
 ```
 
 It will match the following code:
@@ -224,4 +224,4 @@ const foo = () => {
 The first step to compose a rule is to find the target. In this case, we can first use kind: `arrow_function` to find function node. Then we can use other rules to filter candidate nodes that does have return type.
 
 Another trick to write cleaner rule is to use sub-rules as fields.
-Please refer to [composite rule](/guide/rule-config/composite-rule.html#combine-different-rules-as-fields) for more details.
+Please refer to [composite rule](/guide/rule-config/composite-rule.html#combine-different-rules-as-fields) for more details.