docs: fix spacing around "if not exists" brackets

Bruce Momjian <bruce@momjian.us>

Commit: 3da7502cd00ddf8228c9a4a7e4a08725decff99c
Author: Bruce Momjian <bruce@momjian.us>
Date: 2018-03-30T01:25:39Z
Releases: 11.0
docs:  fix spacing around "if not exists" brackets

Reported-by: Fabrízio de Royes Mello

Discussion: https://postgr.es/m/CAFcNs+qDD+QKcF8YCPQnjAxoWN61qY_YdFLB3iQqbWCLSCyY0g@mail.gmail.com

Author: Fabrízio de Royes Mello

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