doc: Fix memory context level in pg_log_backend_memory_contexts() example.

doc: Fix memory context level in pg_log_backend_memory_contexts() example.

Commit d9e03864b6b changed the memory context level numbers shown by
pg_log_backend_memory_contexts() to be 1-based. However, the example in
the documentation was not updated and still used 0-based numbering.

This commit updates the example to match the current 1-based output.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: David Rowley <drowleyml@gmail.com>
Discussion: https://postgr.es/m/1ad6d388-1b43-400d-bec9-36d52f755f74@oss.nttdata.com