pgsql: Fix docs indentation in pgtrgm.sgml

Fix docs indentation in pgtrgm.sgml

5871b884 introduced pg_trgm.word_similarity_threshold GUC, but its documentation
contains wrong indentation.  This commit fixes that.  Backpatch for easier
backpatching of other documentation fixes.

Discussion: https://postgr.es/m/4c735d30-ab59-fc0e-45d8-f90eb5ed3855%402ndquadrant.com
Author: Ian Barwick
Backpatch-through: 9.6

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7c2122f11f0fbc740dd08c0bea3ff1dbf07d4007

Modified Files
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doc/src/sgml/pgtrgm.sgml | 34 +++++++++++++++++-----------------
1 file changed, 17 insertions(+), 17 deletions(-)