pgsql: Change "checkpoint starting" message to use "wal"

Change "checkpoint starting" message to use "wal"

This catches up with the recent renaming of all user-facing mentions
of "xlog" to "wal".

Discussion: https://www.postgresql.org/message-id/flat/20181129084708.GA9562%40msg.credativ.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/60d99797bfb07f8042a8b82c161500f6e4a500ec

Modified Files
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src/backend/access/transam/xlog.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)