Changing variable name in MT WDL to match gatk-workflows

[meganshand]

Small change to match the WDL that's going into gatk-workflows.

[codecov-io]

Codecov Report

Merging #5694 into master will decrease coverage by 69.096%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##              master     #5694        +/-   ##
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- Coverage     87.068%   17.971%   -69.096%     
+ Complexity     31855      8570     -23285     
================================================
  Files           1940      1940                
  Lines         146679    146679                
  Branches       16218     16218                
================================================
- Hits          127710     26360    -101350     
- Misses         13057    117642    +104585     
+ Partials        5912      2677      -3235

Impacted Files	Coverage Δ	Complexity Δ
...ls/variant/writers/GVCFBlockCombiningIterator.java	0% <0%> (-100%)	0% <0%> (-1%)
...nder/tools/copynumber/utils/TagGermlineEvents.java	0% <0%> (-100%)	0% <0%> (-3%)
...r/tools/spark/pathseq/PSBwaArgumentCollection.java	0% <0%> (-100%)	0% <0%> (-1%)
...titute/hellbender/utils/mcmc/DecileCollection.java	0% <0%> (-100%)	0% <0%> (-5%)
...ender/tools/readersplitters/ReadGroupSplitter.java	0% <0%> (-100%)	0% <0%> (-3%)
...ynumber/models/AlleleFractionGlobalParameters.java	0% <0%> (-100%)	0% <0%> (-10%)
...ls/walkers/genotyper/HeterogeneousPloidyModel.java	0% <0%> (-100%)	0% <0%> (-14%)
...rs/vqsr/VariantRecalibratorArgumentCollection.java	0% <0%> (-100%)	0% <0%> (-1%)
...adinstitute/hellbender/engine/ReferenceWalker.java	0% <0%> (-100%)	0% <0%> (-9%)
...ools/funcotator/filtrationRules/ClinVarFilter.java	0% <0%> (-100%)	0% <0%> (-7%)
... and 1639 more

[bshifaw]

Looks good