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

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 8c843fff2d, 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