SQL comment: remove extra word in heading comment

Reported-by: Daniel Gustafsson

Discussion: https://postgr.es/m/431D5BC1-9696-43FA-B54C-39D5503EB753@yesql.se

Backpatch-through: master

src/tutorial/advanced.source

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 -- advanced.sql-
---    Tutorial on advanced more PostgreSQL features
+--    Tutorial on advanced PostgreSQL features
 --
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 -- Copyright (c) 1994, Regents of the University of California