Features: fix FEATURES object (#12407)

```
In [3]: dict(Feature.FEATURES)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[3], line 1
----> 1 dict(Feature.FEATURES)

ValueError: dictionary update sequence element #2 has length 4; 2 is required
```

The object got malformed in
#12327

Author: humitos

readthedocs/projects/models.py

@@ -2040,6 +2040,8 @@ def add_features(sender, **kwargs):
             _(
                 "Build: Don't clean the build directory. Only for Enterprise users with dedicated builders."
             ),
+        ),
+        (
             BUILD_HEALTHCHECK,
             _("Build: Use background cURL healthcheck."),
         ),