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

Documentation/vm/pagemap.rst

 .. _pagemap:
 
-======================================
-pagemap from the Userspace Perspective
-======================================
+=============================
+Examining Process Page Tables
+=============================
 
 pagemap is a new (as of 2.6.25) set of interfaces in the kernel that allow
 userspace programs to examine the page tables and related information by