[exec.await.as] Delete stray "}"

Left over from transforming \tcode into codeblock.

Author: tkoeppe

source/exec.tex

@@ -6769,7 +6769,7 @@
 \end{codeblock}
 and
 \begin{codeblock}
-@\exposid{awaitable-sender}@<decltype((@\exposid{adapted-expr}@)), Promise>}
+@\exposid{awaitable-sender}@<decltype((@\exposid{adapted-expr}@)), Promise>
 \end{codeblock}
 are both satisfied, where \exposid{adapted-expr} is
 \tcode{get_await_completion_adaptor(get_env(expr))(expr)},