CP.2: Fixed a typo in the async example code. (#1030)

span a task -> spawn a task

CppCoreGuidelines.md

@@ -13283,7 +13283,7 @@ Local static variables are a common source of data races.
         // ...
         auto h1 = async([&]{ sort(par, s); });     // spawn a task to sort
         // ...
-        auto h2 = async([&]{ return find_all(buf, sz, pat); });   // span a task to find matches
+        auto h2 = async([&]{ return find_all(buf, sz, pat); });   // spawn a task to find matches
         // ...
     }