[exec.task.scheduler] Fix typo

Author: Andreas-Krug

source/exec.tex

@@ -7072,7 +7072,7 @@
 \pnum
 \tcode{task_scheduler} is a class that models
 \libconcept{scheduler}\iref{exec.sched}.
-Given on object \tcode{s} of type \tcode{task_scheduler}, let
+Given an object \tcode{s} of type \tcode{task_scheduler}, let
 \tcode{\exposid{SCHED}(s)} be the object owned by \tcode{s.\exposid{sch_}}.
 
 \indexlibraryctor{task_scheduler}