Install Guide: Add missing whitespace etc

Add missing space, and make proper sentences with "." and ":".

Change-Id: Ie3b0855c79a44a81d3a7748d6766afb2710b61f0
Closes-Bug: #1259057
backport: havana
(cherry picked from commit 4417b2f7efaab9b3e2d08aeedfb55a1e942b202c)
Merge "Install Guide: Add missing whitespace etc" into stable/havana