docs(trait_checking): import the right function

`is_trait_method` is not even used in this codeblock, whereas `implements_trait` is used but not imported

Author: ada4a

book/src/development/trait_checking.md

@@ -17,7 +17,7 @@ providing the `LateContext` (`cx`), our expression at hand, and
 the symbol of the trait in question:
 
 ```rust
-use clippy_utils::is_trait_method;
+use clippy_utils::implements_trait;
 use rustc_hir::Expr;
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_span::symbol::sym;