docs: Specify language in markdown for syntax highlight
Specify language for code block in docs/Unit-Test-Advice.md
for syntax highlight.
Fixes:
@@ -341,7 +341,7 @@ The main repository has the most comprehensive set of skip abilities. See:
One method is to use the `nix` crate along with some custom macros:
-```
+```rust
#[cfg(test)]
mod tests {
#[allow(unused_macros)]
