[exec.split] Add \libconcept for copyable (cplusplus#7190)

Author: hewillk

source/exec.tex

@@ -3942,7 +3942,7 @@
 where \exposid{shared-wrapper} is an exposition-only class
 that manages the reference count of the \exposid{shared-state} object
 pointed to by sh_state.
-\exposid{shared-wrapper} models copyable
+\exposid{shared-wrapper} models \libconcept{copyable}
 with move operations nulling out the moved-from object,
 copy operations incrementing the reference count
 by calling \tcode{sh_state->\exposid{inc-ref}()}, and