doc: suggest dpkg-buildpackage --no-sign for a smoother experience.

Otherwise it ends with an error message which leads people to think the
package wasn't built.

doc/building.dox

@@ -259,7 +259,7 @@ Building is easy, just change directory to package root, link or copy
 @code{.sh}
 git clone git://github.com/mosra/magnum && cd magnum
 ln -s package/debian .
-dpkg-buildpackage
+dpkg-buildpackage --no-sign
 @endcode
 
 This will compile binary and development packages, which will then appear in a