[dcl.stc] Remove stray whitespace in plural \grammarterm (#1438)

Author: jensmaurer

source/declarations.tex

@@ -401,8 +401,8 @@
 see~\ref{basic.link}.
 \begin{note}
 The \tcode{extern} keyword can also be used in
-\grammarterm{explicit-instantiation}s and
-\grammarterm{linkage-specification}s, but it is not a
+\grammarterm{explicit-instantiation}{s} and
+\grammarterm{linkage-specification}{s}, but it is not a
 \grammarterm{storage-class-specifier} in such contexts.
 \end{note}