Fix description on Query Latency in the doc

[jackjlli]

This PR fixes description on Query Latency in the doc.

@jgutmann

[codecov-io]

Codecov Report

Merging #4111 into master will increase coverage by 0.18%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4111      +/-   ##
============================================
+ Coverage     66.66%   66.85%   +0.18%     
  Complexity       20       20              
============================================
  Files          1037     1037              
  Lines         51425    51425              
  Branches       7192     7192              
============================================
+ Hits          34282    34378      +96     
+ Misses        14787    14676     -111     
- Partials       2356     2371      +15

Impacted Files	Coverage Δ	Complexity Δ
...pinot/core/operator/docidsets/OrBlockDocIdSet.java	84.9% <0%> (-13.21%)	0% <0%> (ø)
...pache/pinot/core/util/SortedRangeIntersection.java	83.82% <0%> (-7.36%)	0% <0%> (ø)
.../impl/dictionary/FloatOnHeapMutableDictionary.java	86.66% <0%> (-6.67%)	0% <0%> (ø)
...e/operator/dociditerators/BitmapDocIdIterator.java	60.71% <0%> (-3.58%)	0% <0%> (ø)
...g/apache/pinot/common/metrics/AbstractMetrics.java	74% <0%> (-2.67%)	0% <0%> (ø)
...ot/core/query/pruner/ColumnValueSegmentPruner.java	85.91% <0%> (-1.41%)	0% <0%> (ø)
...ache/pinot/common/metadata/ZKMetadataProvider.java	72.45% <0%> (-1.2%)	0% <0%> (ø)
...lix/core/realtime/PinotRealtimeSegmentManager.java	73% <0%> (-1%)	0% <0%> (ø)
...ot/common/protocols/SegmentCompletionProtocol.java	92.48% <0%> (-0.58%)	0% <0%> (ø)
...regation/function/customobject/QuantileDigest.java	57.74% <0%> (-0.45%)	0% <0%> (ø)
... and 20 more

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