Fix image pull after a failure

When resuming from a failed pull writer.Truncate() was not
seeking to the proper position in the file. This caused writes to
happen after the previously written content, instead of at the start
of the file.

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

content/helpers.go

@@ -77,7 +77,7 @@ func Copy(ctx context.Context, cw Writer, r io.Reader, size int64, expected dige
 		r, err = seekReader(r, ws.Offset, size)
 		if err != nil {
 			if !isUnseekable(err) {
-				return errors.Wrapf(err, "unabled to resume write to %v", ws.Ref)
+				return errors.Wrapf(err, "unable to resume write to %v", ws.Ref)
 			}
 
 			// reader is unseekable, try to move the writer back to the start.