docs: networking: phy: s/2.5 times faster/2.5 times as fast/

2.5 times faster would be 3.5 Gbps (4.375 Gbaud after 8b/10b encoding).

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20201107220822.1291215-1-j.neuschaefer@gmx.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>