fix transaction extension example

content/200-orm/200-prisma-client/300-client-extensions/120-query.mdx

@@ -267,17 +267,20 @@ You can wrap your extended queries into a [batch transaction](/orm/prisma-client
 The following example extends `findFirst` so that it runs in a batch transaction.
 
 ```ts
-const prisma = new PrismaClient().$extends({
-  query: {
-    user: {
-      // Get the input `args` and a callback to `query`
-      async findFirst({ args, query, operation }) {
-        const [result] = await prisma.$transaction([query(args)]) // wrap the query in a batch transaction, and destructure the result to return an array
-        return result // return the first result found in the array
+const transactionExtension = Prisma.defineExtension((prisma) => 
+  prisma.$extends({
+    query: {
+      user: {
+        // Get the input `args` and a callback to `query`
+        async findFirst({ args, query, operation }) {
+          const [result] = await prisma.$transaction([query(args)]) // wrap the query in a batch transaction, and destructure the result to return an array
+          return result // return the first result found in the array
+        },
       },
     },
-  },
-})
+  })
+)
+const prisma = new PrismaClient().$extends(transactionExtension)
 ```
 
 ## Query extensions versus middlewares