Fix admin API user endpoint documentation examples to use JSON booleans (#19847)

I noticed that the documentation doesn't match reality.

The values from the database are cast to bools at:

- https://github.com/element-hq/synapse/blob/ff034c1f623bc769f1f3bca801592dd42697ea66/synapse/storage/databases/main/registration.py#L424-L439
- https://github.com/element-hq/synapse/blob/ff034c1f623bc769f1f3bca801592dd42697ea66/synapse/storage/databases/main/__init__.py#L330-L344

docs/admin_api/user_admin_api.md

@@ -34,11 +34,11 @@ It returns a JSON body like the following:
         }
     ],
     "avatar_url": "<avatar_url>",  // can be null if not set
-    "is_guest": 0,
-    "admin": 0,
-    "deactivated": 0,
+    "is_guest": false,
+    "admin": false,
+    "deactivated": false,
     "erased": false,
-    "shadow_banned": 0,
+    "shadow_banned": false,
     "creation_ts": 1560432506,
     "last_seen_ts": 1732919539393,
     "appservice_id": null,
@@ -183,24 +183,24 @@ A response body like the following is returned:
     "users": [
         {
             "name": "<user_id1>",
-            "is_guest": 0,
-            "admin": 0,
+            "is_guest": false,
+            "admin": false,
             "user_type": null,
-            "deactivated": 0,
+            "deactivated": false,
             "erased": false,
-            "shadow_banned": 0,
+            "shadow_banned": false,
             "displayname": "<User One>",
             "avatar_url": null,
             "creation_ts": 1560432668000,
             "locked": false
         }, {
             "name": "<user_id2>",
-            "is_guest": 0,
-            "admin": 1,
+            "is_guest": false,
+            "admin": true,
             "user_type": null,
-            "deactivated": 0,
+            "deactivated": false,
             "erased": false,
-            "shadow_banned": 0,
+            "shadow_banned": false,
             "displayname": "<User Two>",
             "avatar_url": "<avatar_url>",
             "creation_ts": 1561550621000,