Paywalls docs: add missing code block markers (#252)

The code snippets for paywalls didn't correctly render from the code
snippets. This is because they were missing the `[block:file]` markers.
This updates the page to add them.

docs_source/🚀 Getting Started/displaying-products/paywalls.md

@@ -124,37 +124,38 @@ Use your own hex codes, select a custom color, or use our color picker to select
 ## How to display a Paywall in your app
 
 1. Option 1: _presentPaywallIfNeeded_ depending on an entitlement
-```swift
+[block:file]
 [
   {
     "language": "swift",
-    "name": "Swift",
-    "file": "code_blocks/🚀 Getting Started/displaying-products/paywalls_1.swift"
+    "name": "",
+    "file": "code_blocks/🚀 Getting Started/displaying-products/paywalls/paywalls_1.swift"
   }
 ]
-```
+[/block]
 
 2. Option 2 _presentPaywallIfNeeded_ with custom logic:
-```swift
+[block:file]
 [
   {
     "language": "swift",
-    "name": "Swift",
-    "file": "code_blocks/🚀 Getting Started/displaying-products/paywalls_2.swift"
+    "name": "",
+    "file": "code_blocks/🚀 Getting Started/displaying-products/paywalls/paywalls_2.swift"
   }
 ]
-```
+[/block]
 
 3. Option 3: present manually:
-```swift
+[block:file]
 [
   {
     "language": "swift",
-    "name": "Swift",
-    "file": "code_blocks/🚀 Getting Started/displaying-products/paywalls_3.swift"
+    "name": "",
+    "file": "code_blocks/🚀 Getting Started/displaying-products/paywalls/paywalls_3.swift"
   }
 ]
-```
+[/block]
+
 ## Limitations
 
 **Platforms** (support for more coming)