fixed typo in 12.3 ARM

#### Summary:

Fix typo in 12.3 ARM R file. (issue #763 )

#### Intended Effect:

Fixes typo so you can pull out the right parameters after sampling.

#### How to Verify:

See R code.

#### Side Effects:

None. Doesn't touch code base.

#### Documentation:

None.

#### Reviewer Suggestions:

Anyone.