Use consistent syntax in profiling vignette

[jgabry]

closes #1136

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Don't switch between target += and ~ syntax.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Jonah Gabry

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.46%. Comparing base (c062d6c) to head (601e59e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1137      +/-   ##
==========================================
- Coverage   87.49%   86.46%   -1.04%     
==========================================
  Files          14       14              
  Lines        5983     5983              
==========================================
- Hits         5235     5173      -62     
- Misses        748      810      +62     

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.46%. Comparing base (c062d6c) to head (601e59e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1137      +/-   ##
==========================================
- Coverage   87.49%   86.46%   -1.04%     
==========================================
  Files          14       14              
  Lines        5983     5983              
==========================================
- Hits         5235     5173      -62     
- Misses        748      810      +62     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[jgabry]

The failure on R-devel is unrelated to this PR. Merging now.