[exec.scope.simple.counting.general] Add missing \tcode

Author: tkoeppe

source/exec.tex

@@ -7968,7 +7968,7 @@
 \exposid{start-join-sender}
 behave as atomic operations\iref{intro.multithread}.
 These operations on a single object of
-type simple_counting_scope appear to occur in a single total order.
+type \tcode{simple_counting_scope} appear to occur in a single total order.
 
 \rSec4[exec.simple.counting.ctor]{Constructor and Destructor}