fix some important std unit tests that were not running due to typo

`#[cfg(features = "std")]` is invalid and always disabled the whole block.

src/lib.rs

@@ -847,7 +847,7 @@ mod tests {
 				mnemonic_str
 			);
 
-			#[cfg(features = "std")]
+			#[cfg(feature = "std")]
 			{
 				assert_eq!(&mnemonic.to_string(), mnemonic_str, "failed vector: {}", mnemonic_str);
 				assert_eq!(