Fix typo on TestNet README

[linked0]

I didn't apply the spell-checked version of README in the last commit. So commit again.

[codecov[bot]]

Codecov Report

Merging #3262 (80ce91c) into v0.x.x (8a3efd7) will increase coverage by 0.89%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           v0.x.x    #3262      +/-   ##
==========================================
+ Coverage   87.81%   88.71%   +0.89%     
==========================================
  Files         164      165       +1     
  Lines       16923    17030     +107     
==========================================
+ Hits        14861    15108     +247     
+ Misses       2062     1922     -140     

Flag	Coverage Δ
integration	35.28% <ø> (?)
unittests	87.75% <ø> (-0.07%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
source/agora/consensus/BlockStorage.d	72.79% <0.00%> (-6.72%)	:arrow_down:
source/agora/consensus/protocol/Nominator.d	91.28% <0.00%> (-0.72%)	:arrow_down:
source/agora/script/Engine.d	97.59% <0.00%> (-0.16%)	:arrow_down:
source/agora/test/Flash.d	95.12% <0.00%> (-0.11%)	:arrow_down:
source/agora/node/main.d	44.85% <0.00%> (ø)
source/agora/consensus/state/Ledger.d	90.93% <0.00%> (+0.33%)	:arrow_up:
source/agora/node/Validator.d	92.96% <0.00%> (+1.55%)	:arrow_up:
source/agora/network/Manager.d	80.65% <0.00%> (+1.96%)	:arrow_up:
source/agora/node/Config.d	71.90% <0.00%> (+2.47%)	:arrow_up:
source/agora/node/FullNode.d	76.16% <0.00%> (+2.78%)	:arrow_up:
... and 8 more

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