[mojo][docs] Minor fix on Unions setter function.

Change-Id: I284e9e3b79f9bdd41320bc0554249a8c90edb583
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4351218
Reviewed-by: Ken Rockot <rockot@google.com>
Auto-Submit: Xinghui Lu <xinghuilu@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/main@{#1118997}

mojo/public/cpp/bindings/README.md

@@ -710,7 +710,7 @@ attempts to access that field will DCHECK:
 
 ```cpp
 ValuePtr value = Value::NewIntValue(42);
-LOG(INFO) << "Value is " << value->string_value();  // DCHECK!
+LOG(INFO) << "Value is " << value->get_string_value();  // DCHECK!
 ```
 
 ### Sending Interfaces Over Interfaces