Fixed typo in README.textile (#26168)

The attribute name was camelCase in the text but in the example it was snake_case.

README.textile

@@ -110,7 +110,7 @@ curl -XGET 'http://localhost:9200/twitter/_search?pretty=true' -H 'Content-Type:
 }'
 </pre>
 
-We can also do range search (the @postDate@ was automatically identified as date)
+We can also do range search (the @post_date@ was automatically identified as date)
 
 <pre>
 curl -XGET 'http://localhost:9200/twitter/_search?pretty=true' -H 'Content-Type: application/json' -d '