Update regex to remove unnecessary /

Author: dwyfrequency

packages/firestore/test/unit/util/bundle.test.ts

@@ -241,7 +241,7 @@ function genericBundleReadingTests(bytesPerRead: number): void {
 
       await expect(
         generateBundleAndParse('{metadata: "no length prefix"}', bytesPerRead)
-      ).to.be.rejectedWith(/(Unexpected end of )(?=.*\JSON\b).*/gi);
+      ).to.be.rejectedWith(/(Unexpected end of )(?=.*JSON\b).*/gi);
 
       await expect(
         generateBundleAndParse(