style: add missing return type on function

Author: Belco90

lib/create-testing-library-rule.ts

@@ -25,7 +25,7 @@ export function createTestingLibraryRule<
       detectionHelpers: Readonly<DetectionHelpers>
     ) => TRuleListener;
   }>
-) {
+): TSESLint.RuleModule<TMessageIds, TOptions, TRuleListener> {
   const { create, ...remainingConfig } = config;
 
   return ESLintUtils.RuleCreator(getDocsUrl)({