[headers] Fix terminology in note

Importable C++ library headers can be imported as header units.
This currently says "as module units".
[module.unit]p1 says that
"A _module unit_ is a translation unit that contains a _module-declaration_."
[module.import]p5 says that the synthetized header unit from an `import <H>;`
is a translation unit that contains no _module-declaration_.

Author: JohelEGP

source/lib-intro.tex

@@ -1093,7 +1093,7 @@
 the \defnadj{importable}{\Cpp{} library headers}.
 \begin{note}
 Importable \Cpp{} library headers can be
-imported as module units\iref{module.import}.
+imported as header units\iref{module.import}.
 \end{note}
 \begin{example}
 \begin{codeblock}