doc: fix wording describing the checkpoint_flush_after GUC

Reported-by: Evan Macbeth

Discussion: https://postgr.es/m/155208475619.1380.12815553062985622271@wrigleys.postgresql.org

Backpatch-through: master

Author: bmomjian

doc/src/sgml/wal.sgml

@@ -602,7 +602,7 @@
 
   <para>
    On Linux and POSIX platforms <xref linkend="guc-checkpoint-flush-after"/>
-   allows to force the OS that pages written by the checkpoint should be
+   allows you to force OS pages written by the checkpoint to be
    flushed to disk after a configurable number of bytes.  Otherwise, these
    pages may be kept in the OS's page cache, inducing a stall when
    <literal>fsync</literal> is issued at the end of a checkpoint.  This setting will