Remove extra ` character (#173)

Fix a typo from the previous commit which I accidentally pushed to
master instead of opening a PR.
diff --git a/lib/io.dart b/lib/io.dart
index 8453ca8..a7bee8f 100644
--- a/lib/io.dart
+++ b/lib/io.dart
@@ -37,7 +37,7 @@
 
   /// The underlying [WebSocket], if this channel has connected.
   ///
-  /// `If the future returned from [WebSocket.connect] has not yet completed, or
+  /// If the future returned from [WebSocket.connect] has not yet completed, or
   /// completed as an error, this will be null.
   WebSocket? get innerWebSocket => _webSocket;