[exec.par.scheduler] Add spaces around operator +

Author: Andreas-Krug

source/exec.tex

@@ -8379,7 +8379,7 @@
 is a proxy \tcode{r} for \tcode{rcvr}
 with base \tcode{system_context_replaceability::bulk_item_receiver_proxy}
 such that
-\tcode{r.execute(i, i+1)} for index \tcode{i}
+\tcode{r.execute(i, i + 1)} for index \tcode{i}
 has effects equivalent to \tcode{f(i, args...)}.
 
 \pnum