Fix comment RefElemLength

[snow54]

Proposed Changes

Give a brief overview of your contribution here in a few sentences.
The comment and the code are inconsistent regarding RefElemLength. Propose to change the comment from 0.1m to 1.0m.

 /*--- Length based parameter for slope limiters uses a default value of
  0.1m ---*/

 RefElemLength = 1.0;

Related Work

Resolve any issues (bug fix or feature request), note any related PRs, or mention interactions with the work of others, if any.
Resolve #1830.

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