fixed syntax error in install guide

The "Install and configure a storage node"
section showed this syntax:

filter = [ "a/sda", "a/sdb/", "r/.*/"]

(one missing / after sda)

The correct syntax is this:

filter = [ "a/sda/", "a/sdb/", "r/.*/" ]

backport: juno

Change-Id: I7b3d4e45cb125979fad3366ebb26c07038088164
Closes-Bug: 1397608