Update doc switcher list for 3.8.0 (GH-16809)

Author: ned-deily

Doc/tools/static/switchers.js

@@ -11,7 +11,7 @@
 
   var all_versions = {
     '3.9': 'dev (3.9)',
-    '3.8': 'pre (3.8)',
+    '3.8': '3.8',
     '3.7': '3.7',
     '3.6': '3.6',
     '3.5': '3.5',