Fix grammar typo: change "an flow" to "a flow"

Co-authored-by: Knerio <96529060+Knerio@users.noreply.github.com>

Author: Copilot

app/graphql/types/errors/flow_validation_error_type.rb

@@ -6,7 +6,7 @@ module Errors
     class FlowValidationErrorType < Types::BaseObject
       graphql_name 'FlowValidationError'
       # rubocop:enable GraphQL/GraphqlName
-      description 'Represents an flow validation error'
+      description 'Represents a flow validation error'
 
       field :details, Errors::ActiveModelErrorType, null: true,
                                                     description: 'Additional details about the validation error'