[istream.unformatted] Add period at end of "Returns"

Author: Andreas-Krug

source/iostreams.tex

@@ -5384,7 +5384,7 @@
 \begin{itemdescr}
 \pnum
 \returns
-\tcode{getline(s, n, widen('\textbackslash n'))}
+\tcode{getline(s, n, widen('\textbackslash n'))}.
 \end{itemdescr}
 
 \indexlibrarymember{ignore}{basic_istream}%