Fix auto-generated ID example format (#29461)

This commit fixes the format of an example auto-generated ID in the docs
where the format misleadingly looked like a UUID rather than a base64
ID.

Author: jasontedor

docs/reference/docs/index_.asciidoc

@@ -229,14 +229,14 @@ The result of the above index operation is:
     },
     "_index" : "twitter",
     "_type" : "_doc",
-    "_id" : "6a8ca01c-7896-48e9-81cc-9f70661fcb32",
+    "_id" : "W0tpsmIBdwcYyG50zbta",
     "_version" : 1,
     "_seq_no" : 0,
     "_primary_term" : 1,
     "result": "created"
 }
 --------------------------------------------------
-// TESTRESPONSE[s/6a8ca01c-7896-48e9-81cc-9f70661fcb32/$body._id/ s/"successful" : 2/"successful" : 1/]
+// TESTRESPONSE[s/W0tpsmIBdwcYyG50zbta/$body._id/ s/"successful" : 2/"successful" : 1/]
 
 [float]
 [[index-routing]]