fix missing quote

Author: fdncred

blog/2023-10-17-nushell_0_86.md

@@ -199,7 +199,7 @@ custom commands. With [#10424](https://github.com/nushell/nushell/pull/10424), d
 overwrite type annotations anymore, i.e. the following is now possible:
 
 ```nushell
-def foo [x: any = null] { print $"$x }
+def foo [x: any = null] { print $"$x" }
 
 foo "string"  # will show `x: string`
 foo null      # will show `x: `