pgsql: Update comment, generation mem contexts have a "keeper" block

Update comment, generation mem contexts have a "keeper" block

The keeper block was introduced in commit 1b0d9aa4f7, but it forgot
to update this comment.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/376c216138c75e161d39767650ea30536f23b482

Modified Files
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src/backend/utils/mmgr/generation.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)