iter-next-slice: make lint adhere to lint message convention

Author: matthiaskrgr

clippy_lints/src/methods/mod.rs

@@ -2280,7 +2280,7 @@ fn lint_iter_next<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>, iter_
                     cx,
                     ITER_NEXT_SLICE,
                     expr.span,
-                    "Using `.iter().next()` on a Slice without end index.",
+                    "using `.iter().next()` on a Slice without end index",
                     "try calling",
                     format!("{}.get({})", snippet_with_applicability(cx, caller_var.span, "..", &mut applicability), start_idx),
                     applicability,
@@ -2299,7 +2299,7 @@ fn lint_iter_next<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>, iter_
             cx,
             ITER_NEXT_SLICE,
             expr.span,
-            "Using `.iter().next()` on an array",
+            "using `.iter().next()` on an array",
             "try calling",
             format!(
                 "{}.get(0)",

tests/ui/iter_next_slice.stderr

@@ -1,24 +1,24 @@
-error: Using `.iter().next()` on an array
+error: using `.iter().next()` on an array
   --> $DIR/iter_next_slice.rs:9:5
    |
 LL |     s.iter().next();
    |     ^^^^^^^^^^^^^^^ help: try calling: `s.get(0)`
    |
    = note: `-D clippy::iter-next-slice` implied by `-D warnings`
 
-error: Using `.iter().next()` on a Slice without end index.
+error: using `.iter().next()` on a Slice without end index
   --> $DIR/iter_next_slice.rs:12:5
    |
 LL |     s[2..].iter().next();
    |     ^^^^^^^^^^^^^^^^^^^^ help: try calling: `s.get(2)`
 
-error: Using `.iter().next()` on a Slice without end index.
+error: using `.iter().next()` on a Slice without end index
   --> $DIR/iter_next_slice.rs:15:5
    |
 LL |     v[5..].iter().next();
    |     ^^^^^^^^^^^^^^^^^^^^ help: try calling: `v.get(5)`
 
-error: Using `.iter().next()` on an array
+error: using `.iter().next()` on an array
   --> $DIR/iter_next_slice.rs:18:5
    |
 LL |     v.iter().next();