Make Mafs line segment color offBlack64 (#1073)

## Summary:
We changed it (accidentally) to offBlack16 in
#1067. This commit fixes it.

Issue: none

Test plan:

View the Mafs segment graph in Storybook (http://localhost:6006/?path=/story/perseus-widgets-interactive-graph--segment-with-mafs-and-locked-points).
Verify that the segment is a darker gray than the gridlines.
<img width="472" alt="Screen Shot 2024-03-13 at 6 11 27 PM" src="https://github.com/Khan/perseus/assets/693920/855b2366-8b97-46cd-9811-4d23d5d73755">

Author: benchristel

Reviewers: jeremywiebe, nedredmond, nishasy, mark-fitzgerald

Required Reviewers:

Approved By: jeremywiebe

Checks: ✅ codecov/project, ✅ codecov/patch, ✅ Upload Coverage, ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Jest Coverage (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ✅ Extract i18n strings (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Cypress (ubuntu-latest, 20.x), ✅ gerald

Pull Request URL: #1073

.changeset/two-flowers-heal.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/perseus": patch
+---
+
+Fix color of Mafs line segments

packages/perseus/src/widgets/interactive-graphs/mafs-styles.css

@@ -23,7 +23,7 @@
 }
 
 .MafsView .movable-segment {
-    --mafs-segment-stroke-color: var(--mafs-line-color);
+    --mafs-segment-stroke-color: rgba(33, 36, 44, 0.64); /* offBlack64 */
     --mafs-segment-stroke-weight: 2px;
 }