fix(apple): Fix undefined variable in config block (#15843)

Example does not compile. Align yielded block variable name in scope and
match Objective-C name

Author: rromanchuk

platform-includes/performance/traces-sampler-as-sampler/apple.mdx

@@ -1,7 +1,7 @@
 ```swift {tabTitle:Swift}
 import Sentry
 
-SentrySDK.start { configureOptions in
+SentrySDK.start { options in
     options.dsn = "___PUBLIC_DSN___"
 
     // To set a uniform sample rate