update dir-spec.txt to match our requirements for Guard flag
In legacy/trac#12690 (moved) we raised the requirements for the Guard flag to be in the top quartile of the network by bandwidth weight, or at least 2000 kilounits.
But the dir-spec.txt still talks about median and 250 kilounits.
We should fix the spec.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information