config-schema: fix schema default for revset-aliases."immutable_heads()"

The default value became definitely outdated when "branches" were renamed
to "bookmarks" in 0.20 (and broke in 0.28).

The actual default of `revset-aliases."immutable_heads()"` is in fact
`builtin_immutable_heads()`, now. However, if one were to want override
to default `immutable_heads()`, `builtin_immutable_heads()` would likely
not be a helpful starting point. Therefore, it may make sense to keep
this "resolved" default value in the schema.

Author: jgreitemann

cli/src/config-schema.json

@@ -590,7 +590,7 @@
                 "immutable_heads()": {
                     "type": "string",
                     "description": "Revisions to consider immutable. Ancestors of these are also considered immutable. The root commit is always considered immutable.",
-                    "default": "present(trunk()) | tags() | untracked_remote_branches()"
+                    "default": "present(trunk()) | tags() | untracked_remote_bookmarks()"
                 }
             },
             "additionalProperties": {