fix: non-LSP-compliant FileSystemWatcher (#10609)

This PR fixes an LSP-non-compliance in the `FileSystemWatcher` that was
introduced in #925.

Closes #10597.

Author: mhuisi

src/Lean/Data/Lsp/Workspace.lean

@@ -29,7 +29,7 @@ structure WorkspaceFolder where
 
 structure FileSystemWatcher where
   globPattern : String
-  kind : Option Nat := none
+  kind? : Option Nat := none
   deriving FromJson, ToJson
 
 namespace FileSystemWatcher