flipped menu open button

[98ZhaoJeffrey]

Notion ticket link

Ticket Name

Implementation description

• flipped the menu open icon on course units page

Steps to test

1. Go to /course and check if the icons are the same but flipped

What should reviewers focus on?

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• I have run the appropriate linter(s)
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR

[carolynzhang18]

[github-actions]

Visit the preview URL for this PR (updated for commit 134267c):

https://extendafamily-7613e--pr80-sidebar-hotfix-rkthnere.web.app

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