Revert default root joint name to "rootJoint"

[jslee02]

The default root joint name was unintentionally changed in #1339, which lead to break several examples that depend on the root joint name. This PR fixes it by reverting the name back.

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@49dd195). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1406   +/-   ##
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  Coverage          ?   57.29%           
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  Files             ?      366           
  Lines             ?    27483           
  Branches          ?        0           
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  Hits              ?    15746           
  Misses            ?    11737           
  Partials          ?        0

Impacted Files	Coverage Δ
dart/utils/urdf/DartLoader.cpp	78.83% <100%> (ø)