Fix documentation example - remove unnecessary mut keyword

Co-authored-by: Byron <[email protected]>

Author: Copilot

gix-diff/src/blob/mod.rs

@@ -29,7 +29,7 @@ pub mod v0_2 {
 /// let after = "fn foo() {\n    let x = 2;\n}\n";
 ///
 /// let input = InternedInput::new(before, after);
-/// let mut diff = diff_with_slider_heuristics(Algorithm::Histogram, &input);
+/// let diff = diff_with_slider_heuristics(Algorithm::Histogram, &input);
 ///
 /// // The diff now has slider heuristics applied
 /// assert_eq!(diff.count_removals(), 1);