[expected.void.obs] Fix index for specialization

Author: JohelEGP

source/utilities.tex

@@ -9182,7 +9182,7 @@
 \tcode{has_value()} is \tcode{true}.
 \end{itemdescr}
 
-\indexlibrarymember{value}{expected}%
+\indexlibrarymember{value}{expected<void>}%
 \begin{itemdecl}
 constexpr void value() const &;
 \end{itemdecl}