add grandchild span to example (#13328)

Author: JoshuaMoelans

platform-includes/performance/enable-manual-instrumentation/native.mdx

@@ -17,16 +17,24 @@ sentry_transaction_context_t *tx_ctx = sentry_transaction_context_new(
 sentry_transaction_t *tx = sentry_transaction_start(tx_ctx, sentry_value_new_null());
 
 // Transactions can have child spans (and those spans can have child spans as well)
-sentry_span_t *span = sentry_transaction_start_child(
+sentry_span_t *child_span = sentry_transaction_start_child(
     tx,
     "child operation",
     "child description"
 );
 
+// Starting a child of a span requires the function `sentry_span_start_child` instead
+sentry_span_t *grandchild_span = sentry_span_start_child(
+    child_span,
+    "grandchild operation",
+    "grandchild description"
+);
+
 // ...
-// (Perform the operation represented by the span/transaction)
+// (Perform the operation represented by the spans/transaction)
 // ...
 
-sentry_span_finish(span); // Mark the span as finished
+sentry_span_finish(grandchild_span); // Mark the spans as finished
+sentry_span_finish(child_span);
 sentry_transaction_finish(tx); // Mark the transaction as finished and send it to Sentry
 ```