[Fix] Update docs for as_update (boolean not supported anymore) (#306)

Fixed documentation to be up to date with latest deprecation of boolean
allowance in as_update.

Author: Guts

docs/configuration.md

@@ -315,7 +315,7 @@ For example, in your `best_article.md` created in 2019, you can write the front-
       - rss:
           date_from_meta:
             as_creation: "date"
-            as_update: false
+            as_update: "git"
             datetime_format: "%Y-%m-%d %H:%M"
             default_timezone: Europe/Paris
     ```
@@ -343,7 +343,7 @@ For example, in your `best_article.md` created in 2019, you can write the front-
       - rss:
           date_from_meta:
             as_creation: "date.created"
-            as_update: false
+            as_update: "git"
             datetime_format: "%Y-%m-%d %H:%M"
             default_timezone: Europe/Paris
     ```

docs/schema.json

@@ -61,7 +61,6 @@
                 },
                 "as_update": {
                   "type": [
-                    "boolean",
                     "string"
                   ]
                 },