pgsql: doc: Fix wording in JIT README

doc: Fix wording in JIT README

Remove superfluous 'is' from sentence.

Author: Yugo Nagata <nagata@sraoss.co.jp>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: /messages/by-id/20250526154412.5f77dfead87af9afc089cc48@sraoss.co.jp

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1f62dbf5f0ae375320b11362e0fccf8ec2766288

Modified Files
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src/backend/jit/README | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)