consistently use "synced folders" terminology

The terminology is "synced folders" not "shared folders" (presumably to
avoid confusion with `vagrant share` which is something completely
different).

website/docs/source/v2/synced-folders/basic_usage.html.md

@@ -66,7 +66,7 @@ Synced folders are automatically setup during `vagrant up` and
 
 ## Disabling
 
-Shared folders can be disabled by adding the `disabled` option to
+Synced folders can be disabled by adding the `disabled` option to
 any definition:
 
 ```ruby