fix: type of QuerySnapshot.docChanges() should be generic

[schmidt-sebastian]

Fixes #1483

[codecov]

Codecov Report

Merging #1484 (e0982c8) into master (685e75c) will increase coverage by 0.02%.
The diff coverage is 99.17%.

@@            Coverage Diff             @@
##           master    #1484      +/-   ##
==========================================
+ Coverage   98.20%   98.22%   +0.02%     
==========================================
  Files          32       32              
  Lines       19502    20052     +550     
  Branches     1363     1390      +27     
==========================================
+ Hits        19152    19697     +545     
- Misses        346      351       +5     
  Partials        4        4              

Impacted Files	Coverage Δ
dev/src/v1beta1/firestore_client.ts	97.93% <98.85%> (+0.19%)	⬆️
dev/src/backoff.ts	98.29% <100.00%> (ø)
dev/src/bulk-writer.ts	100.00% <100.00%> (ø)
dev/src/index.ts	95.45% <100.00%> (+<0.01%)	⬆️
dev/src/query-partition.ts	100.00% <100.00%> (ø)
dev/src/rate-limiter.ts	98.85% <100.00%> (ø)
dev/src/reference.ts	99.21% <100.00%> (ø)
dev/src/timestamp.ts	100.00% <100.00%> (ø)

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