Fix the name of the first WAL segment file, in docs.

Previously the documentation explains that WAL segment files
start at 000000010000000000000000. But the first WAL segment file
that initdb creates is 000000010000000000000001 not
000000010000000000000000. This change was caused by old
commit 8c843ff, but the documentation had not been updated
a long time.

Back-patch to all supported branches.

Author: Fujii Masao
Reviewed-by: David Zhang
Discussion: https://postgr.es/m/CAHGQGwHOmGe2OqGOmp8cOfNVDivq7dbV74L5nUGr+3eVd2CU2Q@mail.gmail.com

Author: MasaoFujii

doc/src/sgml/wal.sgml

@@ -758,7 +758,7 @@
    are described in <filename>access/xlogrecord.h</filename>; the record
    content is dependent on the type of event that is being logged.  Segment
    files are given ever-increasing numbers as names, starting at
-   <filename>000000010000000000000000</filename>.  The numbers do not wrap,
+   <filename>000000010000000000000001</filename>.  The numbers do not wrap,
    but it will take a very, very long time to exhaust the
    available stock of numbers.
   </para>