docs/vm: pagemap: change document title

"pagemap from the Userspace Perspective" is not very descriptive for
unaware readers. Since the document describes how to examine a process page
tables, let's title it "Examining Process Page Tables"

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>