Merge pull request #569 from rescript-association/fix-optional-records-docs-tests

fix tests by making code blocks examples rather than preludes

Author: zth

pages/docs/manual/latest/record.mdx

@@ -162,7 +162,7 @@ ReScript [`v10`](/blog/release-10-0-0#experimental-optional-record-fields) intro
 
 <CodeTab labels={["ReScript", "JS Output"]}>
 
-```res prelude
+```res example
 type person = {
   age: int,
   name?: string
@@ -181,7 +181,7 @@ You can omit any optional fields when creating a record. Not setting an optional
 
 <CodeTab labels={["ReScript", "JS Output"]}>
 
-```res prelude
+```res example
 type person = {
   age: int,
   name?: string