MFde: Minor update to the German documentation set.

doc/de_DE.ISO8859-1/books/handbook/book.sgml fixes only
doc/de_DE.ISO8859-1/books/handbook/advanced-networking/chapter.sgml 1.403 -> 1.413
doc/de_DE.ISO8859-1/books/handbook/introduction/chapter.sgml 1.123 -> 1.126
doc/de_DE.ISO8859-1/books/handbook/virtualization/chapter.sgml fixes only

Obtained from: The FreeBSD German Documentation Project