Update credentials.mdx

Added code for Express in Validating credentials.

docs/pages/getting-started/authentication/credentials.mdx

@@ -540,4 +540,20 @@ export const { handle } = SvelteKitAuth({
 ```
 
 </Code.Svelte>
+
+<Code.Express>
+
+```ts filename="./lib/zod.ts"
+import { z } from "zod";
+export const signInSchema = z.object({
+    email: z.string({ required_error: "Email is required" })
+      .min(1, "Email is required")
+      .email("Invalid email"),
+    password: z.string({ required_error: "Password  is required" })
+      .min(1, "Password is required")
+      .min(8, "Password must be more than 8 characters")
+      .max(32, "Password must be less than 32 characters"),
+});
+```
+</Code.Express>
 </Code>