Update MANUAL_INSTALL.md
Update MANUAL_INSTALL.md

ping needs to have a source interface specified, otherwise the following error will occur: `Failed: no egress interface`

Signed-off-by: Matus Mrekaj <54493619ef998a404e72f066ae6bab852d1f0387@pantheon.tech>

rastislavszabo/vpp,rastislavszabo/vpp,brecode/vpp,rastislavszabo/vpp,rastislavszabo/vpp,rastislavszabo/vpp,brecode/vpp,rastislavszabo/vpp,rastislavszabo/vpp,brecode/vpp,brecode/vpp,brecode/vpp