docs: fix comment in put_slice()

Nitpicky, I know, but renders poorly in the docs.

Author: coryan

src/rt/io.rs

@@ -253,7 +253,7 @@ impl ReadBufCursor<'_> {
         self.buf.remaining()
     }
 
-    /// Transfer bytes into `self`` from `src` and advance the cursor
+    /// Transfer bytes into `self` from `src` and advance the cursor
     /// by the number of bytes written.
     ///
     /// # Panics