doc: fix typo in release notes

Bruce Momjian <bruce@momjian.us>

Commit: 5fbed6b1e1f9e7145679a9f90b5f8b6587f9cc19
Author: Bruce Momjian <bruce@momjian.us>
Date: 2013-10-09T12:44:52Z
Releases: 9.2.6
doc:  fix typo in release notes

Backpatch through 8.4

Per suggestion by Amit Langote

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