docs: fix minor polar documentation issues

docs/content/docs/plugins/polar.mdx

@@ -132,8 +132,10 @@ const auth = betterAuth({
     polar({
       client: polarClient,
       createCustomerOnSignUp: true,
-      getCustomerCreateParams: ({ user, session }, request) => ({
-        myCustomProperty: 123,
+      getCustomerCreateParams: ({ user }, request) => ({
+        metadata: {
+          myCustomProperty: 123,
+        },
       }),
       use: [
         // This is where you add Polar plugins
@@ -156,7 +158,7 @@ const auth = betterAuth({
 
 When `createCustomerOnSignUp` is enabled, a new Polar Customer is automatically created when a new User is added in the Better-Auth Database.
 
-All new customers are created with an associated `externalId`, which is the ID of your User in the Database. This allows us to skip any Polar User mapping in your Database.
+All new customers are created with an associated `externalId`, which is the ID of your User in the Database. This allows us to skip any Polar to User mapping in your Database.
 
 ## Checkout Plugin
 
@@ -211,7 +213,7 @@ await authClient.checkout({
     // Any Polar Product ID can be passed here
     products: ["e651f46d-ac20-4f26-b769-ad088b123df2"],
     // Or, if you setup "products" in the Checkout Config, you can pass the slug
-    slug: 'my-pro-product',
+    slug: 'pro',
     // Reference ID will be saved as `referenceId` in the metadata of the checkout, order & subscription object
     referenceId: organizationId
 });
@@ -306,7 +308,7 @@ const { data: orders } = await authClient.customer.orders.list({
 This method lists the subscriptions associated with authenticated user/customer.
 
 ```typescript
-const { data: subscriptions } = await authClient.customer.orders.list({
+const { data: subscriptions } = await authClient.customer.subscriptions.list({
   query: {
     page: 1,
     limit: 10,
@@ -464,6 +466,3 @@ The plugin supports handlers for all Polar webhook events:
 - `onCustomerUpdated` - Triggered when a customer is updated
 - `onCustomerDeleted` - Triggered when a customer is deleted
 - `onCustomerStateChanged` - Triggered when a customer is created
-
-
-For configuration options and more information, check out [Polar's Better Auth docs](https://docs.polar.sh/integrate/sdk/adapters/better-auth).