[DOCS] "data" not "datums"

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Page: https://www.postgresql.org/docs/9.4/static/datatype-json.html
Description:

In the section
https://www.postgresql.org/docs/9.4/static/datatype-json.html

Sub-section: 
8.14.2. Designing JSON documents effectively

Original text:
"but having a predictable structure makes it easier to write queries that
usefully summarize a set of "documents" (datums) in a table."

Suggested text:
"but having a predictable structure makes it easier to write queries that
usefully summarize a set of "documents" (data) in a table."