Update attrs.md

The current example throws error TS1207: Decorators cannot be applied to multiple get/set accessors of the same name.

I suggest updating documentation inline with advice from TS dev @mhegazy; see microsoft/TypeScript#2249 (comment)

Author: drwsmth

docs/_guide/attrs.md

@@ -258,7 +258,7 @@ class HelloWorldElement extends HTMLElement {
     return 'World' // Used to get the intial value
   }
   // Called whenever `name` changes
-  @attr set dataName(newValue: string) {
+  set dataName(newValue: string) {
     this.textContent = `Hello ${newValue}`
   }
 }