Fix typo in lib.rs (#144)

Author: fbruetting

src/lib.rs

@@ -90,13 +90,10 @@ The `Deref` implementation uses a hidden static variable that is guarded by an a
 
 # Cargo features
 
-This crate provides two cargo features:
+This crate provides one cargo feature:
 
 - `spin_no_std`: This allows using this crate in a no-std environment, by depending on the standalone `spin` crate.
 
-Both features depend on unstable language features, which means
-no guarantees can be made about them in regard to SemVer stability.
-
 */
 
 #![doc(html_root_url = "https://docs.rs/lazy_static/1.3.0")]