Update ledger README reference to trackers

[michaeldiamant]

Summary

Updates ledger README reference to tracker source code.

Since trackers.go does not exist, I assume the appropriate reference is tracker.go.

Test Plan

Since the change only impacts docs, I visually confirmed Markdown rendering.

[codecov-commenter]

Codecov Report

Merging #3540 (2b3baa3) into master (93a372d) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3540      +/-   ##
==========================================
- Coverage   47.57%   47.56%   -0.01%     
==========================================
  Files         370      370              
  Lines       60060    60060              
==========================================
- Hits        28572    28570       -2     
- Misses      28178    28181       +3     
+ Partials     3310     3309       -1     

Impacted Files	Coverage Δ
util/metrics/gauge.go	68.00% <0.00%> (-2.67%)	⬇️
util/metrics/counter.go	78.57% <0.00%> (-2.39%)	⬇️
agreement/cryptoVerifier.go	75.17% <0.00%> (-2.13%)	⬇️
agreement/proposalManager.go	96.07% <0.00%> (-1.97%)	⬇️
catchup/service.go	68.88% <0.00%> (-1.24%)	⬇️
network/requestTracker.go	70.25% <0.00%> (-0.87%)	⬇️
data/transactions/verify/txn.go	44.29% <0.00%> (ø)
network/wsNetwork.go	63.29% <0.00%> (+0.29%)	⬆️
data/abi/abi_type.go	89.57% <0.00%> (+0.94%)	⬆️
ledger/acctupdates.go	66.73% <0.00%> (+1.14%)	⬆️
... and 1 more

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[tzaffi]

LGTM