Address build warning

[aaudiber]

warning: unused variable: `expr`
    --> clippy_lints/src/methods.rs:1888:67
     |
1888 | fn lint_single_char_pattern<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx hir::Expr, arg: &'tcx hir::Expr) {
     |                                                                   ^^^^ help: consider using `_expr` instead
     |
     = note: #[warn(unused_variables)] on by default

[phansch]

Thanks!

[flip1995]

Oh that was my bad! Thanks!