move para about write-env into write-env's itemdescr

a paragraph about _`write-env`_ from P3396R1 got accidentally added outside of _`write-env`_'s `itemdescr`. move it inside.

Author: ericniebler

source/exec.tex

@@ -1893,7 +1893,6 @@
     };
 };
 \end{codeblock}
-\end{itemdescr}
 Invocation of
 \tcode{\exposid{impls-for}<\exposid{write-env-t}>::\exposid{get-env}}
 returns an object \tcode{e} such that
@@ -1907,6 +1906,7 @@
 otherwise, \tcode{e.query(q)} is expression-equivalent
 to \tcode{get_env(rcvr).query(q)}.
 \end{itemize}
+\end{itemdescr}
 
 \rSec2[exec.snd.concepts]{Sender concepts}