[depr.strstream] Fix typography for NTBS in index. (#3942)

Author: jensmaurer

source/future.tex

@@ -1163,7 +1163,7 @@
 Initializes the base class with \tcode{istream(\&sb)} and
 \tcode{sb} with \tcode{strstreambuf(s, 0)}.
 \tcode{s} shall designate the first element of an \ntbs{}.%
-\indextext{NTBS}
+\indextext{NTBS@\ntbs{}}
 \end{itemdescr}
 
 \indexlibraryctor{istrstream}%
@@ -1279,7 +1279,7 @@
 \tcode{(mode \& app) != 0},
 then \tcode{s} shall designate the first element of an array of \tcode{n} elements that
 contains an \ntbs{} whose first element is designated by \tcode{s}.
-\indextext{NTBS}%
+\indextext{NTBS@\ntbs{}}%
 The constructor is
 \tcode{strstreambuf(s, n, s + std::strlen(s))}.\footnote{The function signature
 \indexlibraryglobal{strlen}%