bridge: Fix src_vni argument in man page

bridge: Fix src_vni argument in man page

"SRC VNI" is only one argument and should appear as such. Moreover, this
argument to the src_vni option is documented under three forms: "SRC_VNI",
"SRC VNI" and "VNI" in different places. Consistenly use the simplest form,
"VNI".

Fixes: c5b176e5ba1f ("bridge: fdb: add support for src_vni option")
Signed-off-by: Benjamin Poirier <bpoirier@cumulusnetworks.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>