Correct the Upsert syntax for MongoDB overwrite

[peterbroadhurst]

If a still using Mongo DB (rather than the lighter weight / faster LevelDB option) for receipt stores, they will see the following error when receipts are inserted into the store, after upgrading to https://github.com/hyperledger/firefly-ethconnect/releases/tag/v3.2.2:

[2022-05-27T10:53:11.175-04:00]  INFO Received reply message. requestId='5fcc8464-17f5-4d8a-78ad-da554a7d388d' reqOffset='' type='TransactionFailure': 0xa8c748db42ca12662a4a94f3a23c4123712378e150ef847201a8c72980eb131a
[2022-05-27T10:53:11.175-04:00] ERROR 5fcc8464-17f5-4d8a-78ad-da554a7d388d: addReceipt attempt: 1 failed, err: reflect.Value.Interface: cannot return value obtained from unexported field or method
[2022-05-27T10:53:11.175-04:00]  INFO 5fcc8464-17f5-4d8a-78ad-da554a7d388d: Waiting 0.50s before re-attempt:1 mongo write

[codecov-commenter]

Codecov Report

Merging #220 (178c419) into main (c86cb21) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #220      +/-   ##
==========================================
- Coverage   97.23%   97.20%   -0.03%     
==========================================
  Files          70       70              
  Lines        7661     7660       -1     
==========================================
- Hits         7449     7446       -3     
- Misses        163      164       +1     
- Partials       49       50       +1     

Impacted Files	Coverage Δ
ethconnect/internal/events/eventstream.go	96.99% <0.00%> (-0.51%)	⬇️
ethconnect/internal/rest/mongoreceipts.go	100.00% <0.00%> (ø)

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