Update the intersphinx specification syntax

Closes longturn#2154.

docs/conf.py

@@ -229,7 +229,7 @@
 # -- Options for intersphinx extension ---------------------------------------
 
 # Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {'https://docs.python.org/': None}
+intersphinx_mapping = {'python': ('https://docs.python.org/3', None)}
 
 # -- Options for todo extension ----------------------------------------------