[bibliography] Replace HTTP link with HTTPS link (#6831)

Author: Andreas-Krug

source/back.tex

@@ -25,7 +25,7 @@
 \bibitem{unicode-charmap}
   Unicode Character Mapping Markup Language [online].
   Edited by Mark Davis and Markus Scherer. Revision 5.0.1; 2017-05-31
-  Available from: \url{http://www.unicode.org/reports/tr22/tr22-8.html}
+  Available from: \url{https://www.unicode.org/reports/tr22/tr22-8.html}
 % Literature references.
 \bibitem{cpp-r}
   Bjarne Stroustrup,