Change text for makeindex headings_flag

Question

If I use headings_flag 1 in an mst file with MakeIndex, I get a heading with the title "Numbers". How can I change the text of this string, e.g. in order to translate it into another language?

\documentclass{memoir}
\usepackage{polyglossia}
\setmainlanguage{spanish}
\usepackage{indextools}

\makeindex[name=Índice de pasajes citados,name=numbs,columns=1]

\begin{filecontents}{numbs.mst}
headings_flag 1
\end{filecontents}

\begin{document}

Foo\index[numbs]{11}

\printindex[numbs]

\end{document}