[ES|QL] Update length docs (#116734)

ESQL Update length docs (#116734)

docs/reference/esql/functions/kibana/definition/length.json

@@ -31,7 +31,7 @@
     }
   ],
   "examples" : [
-    "FROM airports\n| KEEP city\n| EVAL fn_length = LENGTH(first_name)"
+    "FROM airports\n| WHERE country == \"India\"\n| KEEP city\n| EVAL fn_length = LENGTH(city)"
   ],
   "preview" : false,
   "snapshot_only" : false

docs/reference/esql/functions/kibana/docs/length.md

@@ -7,7 +7,8 @@ Returns the character length of a string.
 
 ```
 FROM airports
+| WHERE country == "India"
 | KEEP city
-| EVAL fn_length = LENGTH(first_name)
+| EVAL fn_length = LENGTH(city)
 ```
 Note: All strings are in UTF-8, so a single character can use multiple bytes.