Remove <pluginrepository> from maventask.md

[schalkms]

With the next release there's also no need to use the jcenter repo anymore.

[codecov-io]

Codecov Report

Merging #1827 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1827      +/-   ##
============================================
+ Coverage     80.34%   80.34%   +<.01%     
+ Complexity     1960     1950      -10     
============================================
  Files           325      325              
  Lines          5515     5500      -15     
  Branches       1023     1023              
============================================
- Hits           4431     4419      -12     
+ Misses          557      555       -2     
+ Partials        527      526       -1

Impacted Files	Coverage Δ	Complexity Δ
...rturbosch/detekt/formatting/OffsetsToLineColumn.kt	77.77% <0%> (-1.17%)	0% <0%> (ø)
...itlab/arturbosch/detekt/core/ProcessingSettings.kt	76.66% <0%> (-0.08%)	5% <0%> (-10%)

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