BUG: Fix wrong parameter name used to call smoothness_constraint

[carterbox]

Purpose

Fix a runtime error where the wrong keyword is used for a parameter of the smoothness_constraint function.

Approach

Use 'a' instead of 'r'.

Pre-Merge Checklists

Submitter

• Write a helpfully descriptive pull request title.
• Organize changes into logically grouped commits with descriptive commit messages.
• Document all new functions.
• Click 'details' on the readthedocs check to view the updated docs.
• Write tests for new functions or explain why they are not needed.
• Address any complaints from pep8speaks.

Reviewer

• Actually read all of the code.
• Run the new code yourself; the included tests should make this easy.
• Write a summary of the changes as you understand them.
• Thank the submitter.