Doc: networking: Fix typo in dsa

This patch fix typos in Documentation/networking/dsa.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/Documentation/networking/dsa/bcm_sf2.txt b/Documentation/networking/dsa/bcm_sf2.txt
index d999d0c..eba3a24 100644
--- a/Documentation/networking/dsa/bcm_sf2.txt
+++ b/Documentation/networking/dsa/bcm_sf2.txt
@@ -38,7 +38,7 @@
 ======================
 
 The driver is located in drivers/net/dsa/bcm_sf2.c and is implemented as a DSA
-driver; see Documentation/networking/dsa/dsa.txt for details on the subsytem
+driver; see Documentation/networking/dsa/dsa.txt for details on the subsystem
 and what it provides.
 
 The SF2 switch is configured to enable a Broadcom specific 4-bytes switch tag