[Ai] Fix anyOf refdoc (#7066)

The sample code was not correctly enclosed in backticks (`), causing the
code to be incorrectly rendered.

Author: rlazo

firebase-ai/src/main/kotlin/com/google/firebase/ai/type/Schema.kt

@@ -309,15 +309,13 @@ internal constructor(
      *
      * Example: A field that can hold either a simple userID or a more detailed user object.
      *
-     * Schema.anyOf( listOf( Schema.integer(description = "User ID"), Schema.obj(mapOf(
-     *
      * ```
+     * Schema.anyOf( listOf( Schema.integer(description = "User ID"), Schema.obj( mapOf(
      *     "userID" to Schema.integer(description = "User ID"),
      *     "username" to Schema.string(description = "Username")
+     * )))
      * ```
      *
-     * )) )
-     *
      * @param schemas The list of valid schemas which could be here
      */
     @JvmStatic