Fix cargo command for tests

Index: lib/smol_str/tests/tidy.rs
--- lib/smol_str/tests/tidy.rs.orig
+++ lib/smol_str/tests/tidy.rs
@@ -42,7 +42,7 @@ fn check_code_formatting() {
              please run `rustup component add rustfmt --toolchain stable` to install it.",
         );
     }
-    if run("cargo +stable fmt -- --check", &dir).is_err() {
+    if run("cargo fmt -- --check", &dir).is_err() {
         panic!("code is not properly formatted; please format the code by running `cargo fmt`")
     }
 }
