[exec.snd.general] Add missing period (#7172)

Author: Eelis

source/exec.tex

@@ -1229,7 +1229,7 @@
 
 \pnum
 Subclauses \ref{exec.factories} and \ref{exec.adapt} define
-customizable algorithms that return senders
+customizable algorithms that return senders.
 Each algorithm has a default implementation.
 Let \tcode{sndr} be the result of an invocation of such an algorithm or
 an object equal to the result\iref{concepts.equality}, and