markdown: Bump mermaid-rs-renderer to fix flowchart parser panic (#56199)

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Bumps `mermaid-rs-renderer` to
`782b89a7da3f0e91e51f98d00a93acba679be6fb`, which picks up
[1jehuang/mermaid-rs-renderer#95](1jehuang/mermaid-rs-renderer#95),
fixing a panic with partially typed mermaid.

Release Notes:

- Fixed a crash in markdown preview when a mermaid flowchart contained a
partially-typed parallelogram node like `A[/]` or `A[\]`.

Author: agu-z

Cargo.lock

@@ -388,7 +388,7 @@ markdown_preview = { path = "crates/markdown_preview" }
 svg_preview = { path = "crates/svg_preview" }
 media = { path = "crates/media" }
 menu = { path = "crates/menu" }
-mermaid-rs-renderer = { git = "https://github.com/zed-industries/mermaid-rs-renderer", rev = "374db9ead5426697c6c2111151d9f246899bc638", default-features = false }
+mermaid-rs-renderer = { git = "https://github.com/zed-industries/mermaid-rs-renderer", rev = "782b89a7da3f0e91e51f98d00a93acba679be6fb", default-features = false }
 migrator = { path = "crates/migrator" }
 mistral = { path = "crates/mistral" }
 multi_buffer = { path = "crates/multi_buffer" }