docs/vm: remap_file_pages.txt: conert to ReST format

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

Documentation/vm/remap_file_pages.txt

+.. _remap_file_pages:
+
+==============================
+remap_file_pages() system call
+==============================
+
 The remap_file_pages() system call is used to create a nonlinear mapping,
 that is, a mapping in which the pages of the file are mapped into a
 nonsequential order in memory. The advantage of using remap_file_pages()