Update version numbers in Elasticsearch-Py docs (#40355)

docs/python/index.asciidoc

@@ -58,8 +58,8 @@ The recommended way to set your requirements in your `setup.py` or
 
 [source,txt]
 ------------------------------------
-    # Elasticsearch 5.x
-    elasticsearch>=5.0.0,<6.0.0
+    # Elasticsearch 6.x
+    elasticsearch>=6.0.0,<7.0.0
 
     # Elasticsearch 2.x
     elasticsearch2