pgsql: Doc: use CURRENT_USER not USER in plpgsql trigger examples.

Doc: use CURRENT_USER not USER in plpgsql trigger examples.

While these two built-in functions do exactly the same thing,
CURRENT_USER seems preferable to use in documentation examples.
It's easier to look up if the reader is unsure what it is.
Also, this puts these examples in sync with an adjacent example
that already used CURRENT_USER.

Per question from Kirk Parker.

Discussion: https://postgr.es/m/CANwZ8rmN_Eb0h0hoMRS8Feftaik0z89PxVsKg+cP+PctuOq=Qg@mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e187bf06c1e9abafd7f021e9706bf9e11e3dd594

Modified Files
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doc/src/sgml/plpgsql.sgml | 18 +++++++++---------
1 file changed, 9 insertions(+), 9 deletions(-)