[exec.opstate.general] Fix typo

Author: Andreas-Krug

source/exec.tex

@@ -1198,7 +1198,7 @@
 
 \pnum
 The program is ill-formed
-if it performs a copy or move construction or assigment operation on
+if it performs a copy or move construction or assignment operation on
 an operation state object created by connecting a library-provided sender.
 
 \rSec2[exec.opstate.start]{\tcode{execution::start}}