[exec.stopped.opt] Fix indefinite article (cplusplus#7186)

Author: Andreas-Krug

source/exec.tex

@@ -4371,11 +4371,11 @@
 
 \pnum
 \tcode{stopped_as_optional} maps a sender's stopped completion operation
-into a value completion operation as an disengaged \tcode{optional}.
+into a value completion operation as a disengaged \tcode{optional}.
 The sender's value completion operation
 is also converted into an \tcode{optional}.
 The result is a sender that never completes with stopped,
-reporting cancellation by completing with an disengaged \tcode{optional}.
+reporting cancellation by completing with a disengaged \tcode{optional}.
 
 \pnum
 The name \tcode{stopped_as_optional} denotes a pipeable sender adaptor object.