docs(aws-events): Swap previously swapped docs for exists and doesNot…

…Exist (aws#23566)

aws#23469 Addressing this issue. There seems to have accidentally been a documentation swap for these 2 methods somehow. Went ahead and swapped documentation for the 2 so they accurately described the functionality of the 2 methods. 


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packages/@aws-cdk/aws-events/lib/event-pattern.ts

@@ -24,14 +24,14 @@ export class Match implements IResolvable {
   }
 
   /**
-   * Matches when the field is absent from the JSON of the event
+   * Matches when the field is present in the JSON of the event
    */
   public static exists(): string[] {
     return this.fromObjects([{ exists: true }]);
   }
 
   /**
-   * Matches when the field is present in the JSON of the event
+   * Matches when the field is absent from the JSON of the event
    */
   public static doesNotExist(): string[] {
     return this.fromObjects([{ exists: false }]);