[util.smartptr.shared.const] Replace inappropriate "shall"s.

Author: tkoeppe

source/utilities.tex

@@ -10005,8 +10005,8 @@
 
 \pnum
 \ensures
-\tcode{*this} shall contain the old value of
-\tcode{r}. \tcode{r} shall be empty. \tcode{r.get() == nullptr}.
+\tcode{*this} contains the old value of
+\tcode{r}. \tcode{r} is empty, and \tcode{r.get() == nullptr}.
 \end{itemdescr}
 
 \indexlibraryctor{shared_ptr}%