[exec.affine.on] Add missing \placeholder

Author: tkoeppe

source/exec.tex

@@ -6919,7 +6919,7 @@
 \tcode{sch}.
 If the current execution resource is the same as the execution
 resource associated with \tcode{sch}, the completion operation on
-\tcode{\placeholder{out_rcvr}} may be called before \tcode{start(op)} completes.
+\tcode{\placeholder{out_rcvr}} may be called before \tcode{start(\placeholder{op})} completes.
 If scheduling onto \tcode{sch} fails, an error completion on
 \tcode{\placeholder{out_rcvr}} shall be executed on an unspecified execution
 agent.