[intro.refs] Fix clause reference to ISO/IEC 9899.

Author: jensmaurer

source/intro.tex

@@ -73,7 +73,7 @@
 \end{itemize}
 
 \pnum
-The library described in Clause 7 of ISO/IEC 9899:2018
+The library described in ISO/IEC 9899:2018, Clause 7,
 is hereinafter called the
 \defnx{C standard library}{C!standard library}.%
 \footnote{With the qualifications noted in \ref{\firstlibchapter}