Fix a gtk-doc build break caused by commit 623eb5fc7dd2b21f
Fix a gtk-doc build break caused by commit 623eb5fc7dd2b21f

The HTML tags in gtk-doc comments should be properly escaped,
thus gtk-doc doesn't consider them as real HTML tags.
Thanks Fabien (fta) for the notice.

matzipan/evolution-data-server,matzipan/evolution-data-server,Distrotech/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,Distrotech/evolution-data-server,tintou/evolution-data-server,Distrotech/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,Distrotech/evolution-data-server,Distrotech/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server