docs: correct 'Buy Y' to 'Get Y' in promotion instructions (#14990)

Author: NicolasGorga

www/apps/user-guide/app/promotions/create/page.mdx

@@ -280,7 +280,7 @@ If you chose the "Buy X Get Y" promotion type in the first section, fill out the
             - Set the Attribute to Product Category.
             - Set the Operator to "Not In".
             - Set the Value to the "Shirt" category.
-6. In the "What items will the promotion be applied to?" section, you'll specify the Buy Y part (for example, "get 1 free shirt"):
+6. In the "What items will the promotion be applied to?" section, you'll specify the Get Y part (for example, "get 1 free shirt"):
     - You must set the quantity of items that the customer gets out of the promotion. For example, for the "Buy 3 shirts get 1 for free", you set the quantity to one.
     - You must set the products tht the customer gets out of the promotion for free.
     - To add a custom condition, click on the "Add condition" button. For each custom condition:
@@ -390,4 +390,4 @@ The **Limit usage per** field is available from [Medusa Admin v2.11.0](https://g
 
 </Note>
 
-You can now [manage the promotion](../manage/page.mdx) you created.
+You can now [manage the promotion](../manage/page.mdx) you created.