[Bug] Psych::DisallowedClass: Tried to load unspecified class: Symbol

When applying schema element name overrides, it leads to a Psych::DisallowedClass error.

To Reproduce

Apply this diff:

diff --git a/Rakefile b/Rakefile
index 6be5a34..3112c43 100644
--- a/Rakefile
+++ b/Rakefile
@@ -21,7 +21,7 @@ ElasticGraph::Local::RakeTasks.new(
   # Customizes the names of fields generated by ElasticGraph.
   tasks.schema_element_name_overrides = {
     # Example: force inequality operators to be spelled out.
-    # gt: "greaterThan",
+    gt: "greaterThan"
     # gte: "greaterThanOrEqualTo",
     # lt: "lessThan",
     # lte: "lessThanOrEqualTo"

$ be rake
`config/schema/artifacts/datastore_config.yaml` is already up to date.
`config/schema/artifacts/json_schemas.yaml` is already up to date.
`config/schema/artifacts/json_schemas_by_version/v1.yaml` is already up to date.
rake aborted!
Psych::DisallowedClass: Tried to load unspecified class: Symbol (Psych::DisallowedClass)
/Users/myron.marston/.asdf/installs/ruby/4.0.0/bin/bundle:25:in '<main>'