Change scalability paragraph in "Grin for Bitcoiners" doc

[Paouky]

Current:

Pushing that further, between blocks, most outputs end up being spent sooner or later by another input. So all spent outputs can be safely removed. And the whole blockchain can be stored, downloaded and fully verified in just a few gigabytes or less (assuming a number of transactions similar to bitcoin).

This means that the Grin blockchain scales with the number of users (unspent outputs), not the number of transactions. At the moment, there is one caveat to that: a small piece of data (called a kernel, about 100 bytes) needs to stay around for each transaction. But we're working on optimizing that as well.

Suggested by tromp:

Pushing that further, between blocks, most outputs end up being spent sooner or later as a transaction input. So all spent outputs can be safely removed.
That leaves only the kernel of a transaction as having a lasting impact (at around 100 bytes) on the size of the blockchain.
Asymptotically, a Mimblewimble blockchain grows at roughly one quarter the rate of a bitcoin blockchain, while offering much better privacy.

[Paouky]

#49