fix typos in design-file-based-storage.rst doc
fix typos in design-file-based-storage.rst doc

Fix typos in doc/design-file-based-storage.rst.

Signed-off-by: Gangbiao Liu <9a4db330741fc51aa2f9bd2adf797e098cd069c7@gmail.com>
Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>

leshchevds/ganeti,onponomarev/ganeti,andir/ganeti,bitemyapp/ganeti,onponomarev/ganeti,andir/ganeti,yiannist/ganeti,mbakke/ganeti,grnet/snf-ganeti,leshchevds/ganeti,yiannist/ganeti,bitemyapp/ganeti,grnet/snf-ganeti,andir/ganeti,ganeti/ganeti,dimara/ganeti,yiannist/ganeti,mbakke/ganeti,ganeti/ganeti,dimara/ganeti,mbakke/ganeti,ganeti/ganeti,leshchevds/ganeti