[12.x] Fix Arr::push example to show the actual result

[AhmedAlaa4611]

Description

This PR updates the Arr::push documentation example to reflect the actual return value. Since Arr::push always appends the given value into an array, the corrected example should use the array form.

When I run this code I get:

array:1 [
  "office" => array:1 [
    "furniture" => array:1 [
      0 => "Desk"
    ]
  ]
]