Documentation: networking: fix ASCII art in switchdev.txt

Fix ASCII art in Documentation/networking/switchdev.txt:

Change non-ASCII "spaces" to ASCII spaces.

Change 2 erroneous '+' characters in ASCII art to '-' (at the '*'
characters below):

line 32:
                     +--+----+----+----+-*--+----+---+  +-----+-----+
line 41:
                     +--------------+---*------------+

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>