[basic.namespace] Properly mark grammar non-terminal (#3885)

Author: sdkrystian

source/declarations.tex

@@ -7010,7 +7010,7 @@
 \pnum
 \begin{note}
 A namespace name with external linkage is exported
-if any of its \term{namespace-definition}{s} is exported,
+if any of its \grammarterm{namespace-definition}{s} is exported,
 or if it contains any
 \grammarterm{export-declaration}{s}\iref{module.interface}.
 A namespace is never attached to a module,