[re.results.state] Fix bad index for match_result::ready (#1531)

Correctly index match_result::ready, instead of the non-existent "match_result::status".

Author: Alisdair Meredith

source/regex.tex

@@ -2655,7 +2655,7 @@
 
 \rSec2[re.results.state]{\tcode{match_results} state}
 
-\indexlibrarymember{match_results}{state}%
+\indexlibrarymember{match_results}{ready}%
 \begin{itemdecl}
 bool ready() const;
 \end{itemdecl}