[headers] Fix note about importing library headers

Importable library headers are not module units,
because they do not have a module-declaration.

Author: JohelEGP

source/lib-intro.tex

@@ -1099,7 +1099,7 @@
 the \defnadj{importable}{\Cpp{} library headers}.
 \begin{note}
 Importable \Cpp{} library headers can be
-imported as module units\iref{module.import}.
+imported\iref{module.import}.
 \end{note}
 \begin{example}
 \begin{codeblock}