Fix broken license link

[benwicks]

I found a broken link in the CONTRIBUTING docs. The link was referencing the LICENSE document on master (which doesn't exist currently). I fixed it to point to the correct document on the 2.x branch.

[codecov]

Codecov Report

Merging #5662 into 2.x will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5662      +/-   ##
============================================
- Coverage     96.24%   96.24%   -0.01%     
+ Complexity     5853     5850       -3     
============================================
  Files           635      635              
  Lines         41651    41651              
  Branches       5768     5768              
============================================
- Hits          40089    40085       -4     
- Misses          616      618       +2     
- Partials        946      948       +2

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/observable/ObservablePublish.java	92.98% <0%> (-3.51%)	11% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.23% <0%> (-3.39%)	2% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-3.23%)	9% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	60.28% <0%> (-2.84%)	32% <0%> (-2%)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	93.83% <0%> (-2.21%)	11% <0%> (ø)
.../main/java/io/reactivex/subjects/MaybeSubject.java	97.82% <0%> (-2.18%)	47% <0%> (-1%)
...ex/internal/subscribers/InnerQueuedSubscriber.java	96.07% <0%> (-1.97%)	18% <0%> (-1%)
...vex/internal/subscribers/QueueDrainSubscriber.java	50% <0%> (-1.57%)	15% <0%> (-1%)
...ternal/operators/completable/CompletableMerge.java	97.61% <0%> (-1.2%)	2% <0%> (ø)
... and 19 more

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