Update a link to Micrometer's docs for the executor metrics

[bsideup]

Since micrometer-metrics/micrometer-docs#97 is closed now,
We should point the users to that page.

[codecov-io]

Codecov Report

Merging #1817 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1817      +/-   ##
============================================
+ Coverage     81.81%   81.81%   +<.01%     
+ Complexity     3845     3844       -1     
============================================
  Files           367      367              
  Lines         30225    30225              
  Branches       5630     5630              
============================================
+ Hits          24728    24730       +2     
  Misses         3935     3935              
+ Partials       1562     1560       -2

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/reactor/core/scheduler/SchedulerTask.java	73.21% <0%> (-3.58%)	16% <0%> (-1%)
...va/reactor/core/publisher/ParallelMergeReduce.java	69.67% <0%> (-3.28%)	3% <0%> (ø)
...ain/java/reactor/core/publisher/FluxConcatMap.java	89.75% <0%> (-0.28%)	7% <0%> (ø)
...ain/java/reactor/core/publisher/FluxPublishOn.java	86.59% <0%> (-0.21%)	6% <0%> (ø)
...c/main/java/reactor/core/publisher/FluxReplay.java	84.41% <0%> (+0.15%)	25% <0%> (ø)	⬇️
.../main/java/reactor/core/publisher/FluxFlatMap.java	94.98% <0%> (+0.19%)	13% <0%> (ø)	⬇️
...eactor/core/publisher/ParallelMergeSequential.java	81.34% <0%> (+1.03%)	7% <0%> (ø)	⬇️
.../java/reactor/core/publisher/BlockingIterable.java	79.16% <0%> (+1.04%)	7% <0%> (ø)	⬇️
...r-core/src/main/java/reactor/core/Disposables.java	90.18% <0%> (+1.22%)	21% <0%> (ø)	⬇️
...actor/core/publisher/FluxOnBackpressureLatest.java	83.67% <0%> (+3.06%)	2% <0%> (ø)	⬇️

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