docs: networking: phy: Improve placement of parenthesis

"either" is outside the parentheses, so the matching "or" should be too.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>