docs: fix a link of files API in README.md (#1113)
docs: fix a link of files API in README.md (#1113)

The original link of files API was https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/FILES. But it should be changed to https://github.com/ipfs/interface-ipfs-core/blob/master/SPEC/FILES.md.

Signed-off-by: Sangwon Hong <78db7273bea9f1fe8c8df5e71e27a09754c02420@gmail.com>
ipfs/node-ipfs