Fixed comment of as_mut_slice (libstd/option.rs)

The old comment did not describe the function correctly

src/libstd/option.rs

@@ -104,7 +104,7 @@ pub fn as_slice<'r>(&'r self) -> &'r [T] {
         }
     }
 
-    /// Convert from `Option<T>` to `&[T]` (without copying)
+    /// Convert from `Option<T>` to `&mut [T]` (without copying)
     #[inline]
     pub fn as_mut_slice<'r>(&'r mut self) -> &'r mut [T] {
         match *self {