docs: break up long lines in vmxnet3.rst to avoid line wrap

Although it does not make a difference for the formatter with long lines, it
is easier to view and read the raw text with conventional editor when there no
line wrap. Just break up the long lines to 80 characters or less.

Change-Id: Ia916638e974abd19b8d19aee4dd72ff6a243ca48
Signed-off-by: Steven Luong <sluong@cisco.com>