fix: YAML formatting in backward-compatibility guide (#1227)

Author: gildesmarais

src/content/docs/ruby-gem/guides/backward-compatibility.mdx

@@ -30,8 +30,8 @@ Update any occurrences of `pubDate` or `updated` in your `selectors` block to `p
 # Current & Required
 
 selectors:
-published_at:
-selector: ".date"
+  published_at:
+    selector: ".date"
 `}
 lang="yaml"
 />
@@ -53,9 +53,9 @@ lang="yaml"
 
 strategy: browserless
 headers:
-User-Agent: "CustomAgent/1.0"
+  User-Agent: "CustomAgent/1.0"
 channel:
-url: "https://example.com/articles"
+  url: "https://example.com/articles"
 `}
 lang="yaml"
 />