Fixes #1985 : Update README to refer the latest documentation

[valfirst]

Fixes #1985

[TimvdLippe]

I think we could even use http://javadoc.io/page/org.mockito/mockito-core/latest/org/mockito/Mockito.html to make it work for Mockito 4. Could you use that link?

[codecov-commenter]

Codecov Report

Merging #1986 into release/3.x will decrease coverage by 0.02%.
The diff coverage is n/a.

@@                Coverage Diff                @@
##             release/3.x    #1986      +/-   ##
=================================================
- Coverage          85.28%   85.25%   -0.03%     
  Complexity          2601     2601              
=================================================
  Files                322      322              
  Lines               7510     7510              
  Branches             898      898              
=================================================
- Hits                6405     6403       -2     
  Misses               869      869              
- Partials             236      238       +2     

Impacted Files	Coverage Δ	Complexity Δ
...to/internal/util/concurrent/WeakConcurrentMap.java	39.36% <0.00%> (-2.13%)	11.00% <0.00%> (ø%)

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[TimvdLippe]

Thanks!