Clarify that OData operator for "not equals" is ne, not neq

[matthew-white]

For getodk/central#503, I'm currently planning to use the OData "not equals" operator in the $filter query parameter, something like __system/conflict ne null. While looking into that, I noticed that the API docs say that neq is the OData operator. However, the operator is actually ne: https://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part2-url-conventions.html#sec_NotEquals. I updated a test to confirm that.

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• run make test-full and confirmed all checks still pass OR confirm CircleCI build passes
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[sadiqkhoja]

ha! I have used not equal like this: $filter=not __system/reviewState eq \'rejected\' in a test :)