pgsql: Fix typos in comments and doc

Fix typos in comments and doc

overriden -> overridden

The misspelling in create_extension.sgml was introduced in b67aaf2,
so no need to backpatch.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/62e2ddd4ca47756372f2788256f445bba6bb492c

Modified Files
--------------
doc/src/sgml/ref/create_extension.sgml |    2 +-
src/backend/bootstrap/bootstrap.c      |    2 +-
src/makefiles/Makefile.win32           |    2 +-
3 files changed, 3 insertions(+), 3 deletions(-)