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ARROW-6820: [Format] Update Map type child to "entries"

This changes the Map type child field name from "entry" to "entries." This is more appropriate because the field is a list of key/value entries, and the C++ and Java implementations already use "entries." This name is only a recommendation for field names and specific names for Map are not enforced.

Closes #5821 from BryanCutler/format-update-Map-entries-ARROW-6820 and squashes the following commits:

f857234 <Bryan Cutler> Update format Schema for MapType with child entries

Authored-by: Bryan Cutler <cutlerb@gmail.com>
Signed-off-by: Bryan Cutler <cutlerb@gmail.com>