[coro.generator.promise] Use "equivalent to throw, ..."

Our style guide says that "equivalent to <expression>" is the
appropriate style for an expression of type `void`, which is the case
for `throw`. This avoids the awkward situation of having "equivalent
to: `throws;`" appear in the middle of a paragraph without proper
terminal punctuation.

Fixes NB US 119 (C++23 CD).

Author: tkoeppe

source/ranges.tex

@@ -15420,8 +15420,8 @@
 If the handle referring to the coroutine
 whose promise object is \tcode{*this}
 is the sole element of \tcode{*x.\exposid{active_}},
-equivalent to: \tcode{throw;}
-Otherwise, assigns \tcode{current_exception()} to \exposid{except_}.
+equivalent to \tcode{throw},
+otherwise, assigns \tcode{current_exception()} to \exposid{except_}.
 \end{itemdescr}
 
 \indexlibrarymember{operator new}{generator::promise_type}%