docs: backtick IOSurface in README to fix clippy doc_markdown lint

The README is compiled as crate docs via include_str!, so clippy's
doc_markdown lint runs over it under -D warnings. The bare 'IOSurface'
identifier in the 3.0 migration note tripped the lint and failed the
Lint CI job on main.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 1313

README.md

@@ -455,7 +455,7 @@ Highlights by major version:
   `Send + Sync`; `PixelFormat` and `SCStreamErrorCode` became
   `#[non_exhaustive]`; `PixelFormat` gained `Unknown(FourCharCode)`; every
   `macos_*` Cargo feature now propagates to the Swift bridge build.
-- **3.0** — foundation types (Core Graphics / Core Media / IOSurface / Core
+- **3.0** — foundation types (Core Graphics / Core Media / `IOSurface` / Core
   Video) moved onto the shared [`apple-cf`](https://crates.io/crates/apple-cf)
   / [`apple-metal`](https://crates.io/crates/apple-metal) crates as re-exports;
   ScreenCaptureKit-specific `CMSampleBuffer` accessors moved to the