Fix config serializer regex (#10487)

app/lib/proof_provider/keybase/config_serializer.rb

@@ -34,7 +34,7 @@ class ProofProvider::Keybase::ConfigSerializer < ActiveModel::Serializer
   end
 
   def username
-    { min: 1, max: 30, re: '[a-z0-9_]+([a-z0-9_\.-]+[a-z0-9_]+)?' }
+    { min: 1, max: 30, re: '[a-z0-9_]+([a-z0-9_.-]+[a-z0-9_]+)?' }
   end
 
   def prefill_url