[docs] LibraryEvolution: s/proposal/document/
[docs] LibraryEvolution: s/proposal/document/

"Proposal" refers to something going through the Swift Evolution Process,
which this won't as such. (Although it does have many pieces affected by
proposals, as described at the end of the document.)

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift