patch 8.0.1385: Python 3.5 is getting old

Problem:    Python 3.5 is getting old.
Solution:   Make Python 3.6 the default. (Ken Takata, closes #2429)
diff --git a/runtime/doc/if_pyth.txt b/runtime/doc/if_pyth.txt
index a870a50..b4577fc 100644
--- a/runtime/doc/if_pyth.txt
+++ b/runtime/doc/if_pyth.txt
@@ -736,8 +736,8 @@
 
 The name of the DLL should match the Python version Vim was compiled with.
 Currently the name for Python 2 is "python27.dll", that is for Python 2.7.
-That is the default value for 'pythondll'.  For Python 3 it is python35.dll
-(Python 3.5).  To know for sure edit "gvim.exe" and search for
+That is the default value for 'pythondll'.  For Python 3 it is python36.dll
+(Python 3.6).  To know for sure edit "gvim.exe" and search for
 "python\d*.dll\c".