Correct spelling mistake in slip-0015.md (#320)

'given' is mispelled as 'fiven'

slip-0015.md

@@ -39,7 +39,7 @@ We first derive a *master key* from hardware device itself, which is shared for
 
 We then derive *account key* for every account. This key is a string -- because of the stated goal 3., we want to be able to import it into third party applications without HD wallets.
 
-From the account key, we derive both a filename and a symmetric encryption key. We then save the metadata to the fiven file, in an encrypted JSON.
+From the account key, we derive both a filename and a symmetric encryption key. We then save the metadata to the given file, in an encrypted JSON.
 
 ## Design details