Fix typo in Rx2 readme

[aballano]

No description provided.

[codecov]

Codecov Report

Merging #865 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #865      +/-   ##
============================================
- Coverage     45.38%   45.35%   -0.03%     
  Complexity      652      652              
============================================
  Files           302      302              
  Lines          7756     7756              
  Branches        833      833              
============================================
- Hits           3520     3518       -2     
  Misses         3932     3932              
- Partials        304      306       +2

Impacted Files	Coverage Δ	Complexity Δ
...ics/src/main/kotlin/arrow/optics/instances/mapk.kt	87.5% <0%> (-4.17%)	0% <0%> (ø)
...tics/src/main/kotlin/arrow/optics/instances/map.kt	91.17% <0%> (-2.95%)	0% <0%> (ø)

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