Rename some "python3" symbols to "py3", as the command name.
Documentation updates.
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index 3e83ef7..e45796f 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -741,7 +741,7 @@
 			RANGE|EXTRA|NEEDARG|CMDWIN),
 EX(CMD_pyfile,		"pyfile",	ex_pyfile,
 			RANGE|FILE1|NEEDARG|CMDWIN),
-EX(CMD_python3,		"py3",	ex_python3,
+EX(CMD_py3,		"py3",		ex_py3,
 			RANGE|EXTRA|NEEDARG|CMDWIN),
 EX(CMD_py3file,		"py3file",	ex_py3file,
 			RANGE|FILE1|NEEDARG|CMDWIN),