fix vmodtool generated RST headers
fix vmodtool generated RST headers

Neither the vmod_ default $Prefix not an author-supplied $Prefix has
got anything to do with the actual name of the vmod.

gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache