[trivial,doc] Fix memory consistency model in comment

Author: skeees

src/scheduler.h

@@ -110,7 +110,7 @@ class SingleThreadedSchedulerClient {
 
     /**
      * Add a callback to be executed. Callbacks are executed serially
-     * and memory is sequentially consistent between callback executions.
+     * and memory is release-acquire consistent between callback executions.
      * Practially, this means that callbacks can behave as if they are executed
      * in order by a single thread.
      */