sector sealing specifics link to lotus source on github leads nowhere

[acud]

Hi there,

Under section 2.6.1.2 Sector Sealing, at the end of the section there's a link to github:

The Lotus-specific set of functions applied to the sealing of a sector can be found here.

The link leads to a 404. Looking at the storage folder, the file does not exist anymore. Furthermore, in the sealer folder the architecture appears to have changed significantly such that such a file doesn't exist there either. It might be useful to drop the sentence or at least point into the correct place(s).