Switch from std to var in arm tutorial (#486)

## Description

<!-- Provide a brief description of the PR's purpose here. -->

The arm tutorial currently sets the objective / fitness to be the
negative standard deviation. However, the original benchmark used the
negative _variance_. This PR switches to the variance and re-runs the
notebook accordingly.

## Status

- [x] I have read the guidelines in

[CONTRIBUTING.md](https://github.com/icaros-usc/pyribs/blob/master/CONTRIBUTING.md)
- [x] I have formatted my code using `yapf`
- [x] I have tested my code by running `pytest`
- [x] I have linted my code with `pylint`
- [x] I have added a one-line description of my change to the changelog
in
      `HISTORY.md`
- [x] This PR is ready to go

HISTORY.md

@@ -20,6 +20,7 @@
 
 #### Documentation
 
+- Switch from std to var in arm tutorial ({pr}`486`)
 - Fix documentation conf for readthedocs deprecations ({pr}`485`)
 - Add novelty search with CMA-ES to sphere example ({pr}`478`, {pr}`482`)