[exec.sync.wait.var] Fix wording

A misapplication of P3557R3.

Author: tkoeppe

source/exec.tex

@@ -5794,8 +5794,7 @@
 \end{itemize}
 
 \pnum
-The expression \tcode{sync_wait_with_variant.apply_sender(sndr)} is ill-formed.
-Otherwise, it is equivalent to:
+The expression \tcode{sync_wait_with_variant.apply_sender(sndr)} is equivalent to:
 \begin{codeblock}
 using result_type = @\exposid{sync-wait-with-variant-result-type}@<Sndr>;
 if (auto opt_value = sync_wait(into_variant(sndr))) {