docs: add Content-Type header for JWT authentication (#12513)

### What?
Fix docs fragment about JWT strategy authentication

### Why?
The example in docs doesn't work out of the box
<img width="535" alt="image"
src="https://github.com/user-attachments/assets/ae62b89e-25bd-4d50-b64f-f0edb4f40ca7"
/>

Solution is to set `Content-Type: application/json`
<img width="819" alt="image"
src="https://github.com/user-attachments/assets/4e645576-071d-436d-a5e2-eaa9e218f855"
/>

docs/authentication/jwt.mdx

@@ -23,6 +23,9 @@ Example:
 ```ts
 const user = await fetch('http://localhost:3000/api/users/login', {
   method: 'POST',
+  headers: {
+    'Content-Type': 'application/json',
+  },
   body: JSON.stringify({
     email: 'dev@payloadcms.com',
     password: 'password',