fixup: add missing \libconcept

Co-authored-by: Jan Schultke <[email protected]>

Author: dietmarkuehl

source/exec.tex

@@ -5735,7 +5735,7 @@
 returned from \tcode{affine_on(sndr, sch)} or one equal to such,
 and let \tcode{OutSndr} be the type \tcode{decltype((out_sndr))}.
 Let \tcode{out_rcvr} be a subexpression denoting a receiver that
-has an environment of type \tcode{Env} such that \tcode{sender_in<OutSndr, Env>}
+has an environment of type \tcode{Env} such that \tcode{\libconcept{sender_in}<OutSndr, Env>}
 is \tcode{true}.  Let \tcode{op} be an lvalue referring to
 the operation state that results from connecting \tcode{out_sndr}
 to \tcode{out_rcvr}. Calling \tcode{start(op)} will start \tcode{sndr}