Fix triangle diagram typos and improve caption clarity

[aashu2006]

This PR fixes two errors in the triangle diagram alt text and improves the caption wording for clarity and accessibility in the variables-and-change tutorial.

Changes:

• Fix typo: "triangel" → "triangle"
• Fix incorrect coordinate in the example syntax: x1, y2 → x1, y1
• Simplify caption language for clearer screen reader output

This is a small, scoped documentation improvement with no changes to tutorial structure or examples.