doc: Fix style of description for pg_buffercache_numa.os_page_num

Extracted from a larger patch by the same author.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/Z/fFA2heH6lpSLlt@ip-10-97-1-34.eu-west-3.compute.internal

Author: michaelpq

doc/src/sgml/pgbuffercache.sgml

@@ -278,7 +278,7 @@
        <structfield>os_page_num</structfield> <type>bigint</type>
       </para>
       <para>
-       number of OS memory page for this buffer
+       Number of OS memory page for this buffer
       </para></entry>
      </row>