pgsql: worker_spi: Rename custom wait event to "WorkerSpiMain"

worker_spi: Rename custom wait event to "WorkerSpiMain"

This naming is more consistent with all the other user-facing wait event
strings.  Other in-core modules will use the same naming convention, so
let's be consistent here as well.

Extracted from a larger patch by the same author.

Author: Masahiro Ikeda
Discussion: https://postgr.es/m/197bce267fa691a0ac62c86c4ab904c4@oss.nttdata.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c8e318b1b8d9b699a96b96db861b362550456108

Modified Files
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doc/src/sgml/xfunc.sgml                         |  6 +++---
src/test/modules/worker_spi/t/001_worker_spi.pl | 12 ++++++------
src/test/modules/worker_spi/worker_spi.c        |  2 +-
3 files changed, 10 insertions(+), 10 deletions(-)