Fix a typo (#300)

src/verifying.rs

@@ -170,7 +170,7 @@ impl VerifyingKey {
 
     /// Returns whether this is a _weak_ public key, i.e., if this public key has low order.
     ///
-    /// A weak public key can be used to generate a siganture that's valid for almost every
+    /// A weak public key can be used to generate a signature that's valid for almost every
     /// message. [`Self::verify_strict`] denies weak keys, but if you want to check for this
     /// property before verification, then use this method.
     pub fn is_weak(&self) -> bool {