Fix non-standard deskop file

desktop-file-validate /home/rpmaker/rpmbuild/BUILDROOT/linkchecker-8.6-1.fc21.i386/usr/share/applications/linkchecker-gui.desktop:

/home/rpmaker/rpmbuild/BUILDROOT/linkchecker-8.6-1.fc21.i386/usr/share/applications/linkchecker-gui.desktop: error: value "Network;WebDevelopment" for string list key "Categories" in group "Desktop Entry" does not have a semicolon (';') as trailing character