chore(psl): use single-flag bitflags form for the hidden set

Apply the review suggestion: a single-variant set does not need the
brace form.

Signed-off-by: Alexey Orlenko's AI Agent <robot@aqrln.net>

Author: tensordreams

psl/psl-core/src/common/preview_features.rs

@@ -232,7 +232,7 @@ impl<'a> FeatureMapWithProvider<'a> {
                 | TransactionApi
                 | UncheckedScalarInputs
             }),
-            hidden: enumflags2::make_bitflags!(PreviewFeature::{ReactNative}),
+            hidden: enumflags2::make_bitflags!(PreviewFeature::ReactNative),
         };
 
         Self {