Use "unlink" instead of "destroy" in docs (#39)

["destroy" was renamed to "unlink"][0] in the code. This updates the
docs to reflect that change.

[0]: random-access-storage/random-access-storage@bcbe0d6

README.md

@@ -79,7 +79,7 @@ Stat the storage. Should return an object with useful information about the unde
 
 Close the underlying file descriptor.
 
-#### `file.destroy([callback])`
+#### `file.unlink([callback])`
 
 Unlink the underlying file.