Reflects the latest API version

The API.md document should reflect the currently API version and, for the current version, Mastodon's version is always present. Therefore, the version attribute it's not nullable.

Using-the-API/API.md

@@ -593,7 +593,7 @@ Note: Some attributes in the entity payload can have ``null`` value and are mark
 | `title`                  | The instance's title                                                     | no       |
 | `description`            | A description for the instance                                           | no       |
 | `email`                  | An email address which can be used to contact the instance administrator | no       |
-| `version`                | The Mastodon version used by instance (as of version 1.3).               | yes      |
+| `version`                | The Mastodon version used by instance.                                   | no       |
 
 ### Mention