pgsql: Update the description of type of check_option reloption in docs

Update the description of type of check_option reloption in docs.

Commit 773df883e8f7 changed the type of check_option reloption
from string to enum. But it forgot to update the description of
the type in the documentation.

Author: Atsushi Torikoshi
Discussion: https://postgr.es/m/CACZ0uYFvHF4n6yxF390YZgr4Q0Z0c2w0ihu=DLb8ipNOnNcqzQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1558413432e3dce941af550f1ad0d41790ec6fba

Modified Files
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doc/src/sgml/ref/alter_view.sgml  | 2 +-
doc/src/sgml/ref/create_view.sgml | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)