fix(docs): `use-flakehub` inconsistency

[Malix-Labs]

• magic-nix-cache-action/action.yml

  Lines 24 to 31 in cec65ff

  	use-flakehub:
  	description: |
  	Whether to upload build results to FlakeHub Cache.
  	Set to "no-preference" or null to have FlakeHub Cache turn on opportunistically (default).
  	Set to "enabled" or true to explicitly request FlakeHub Cache.
  	Set to "disabled" or false to explicitly disable FlakeHub Cache.
  	default: null
  	required: false

  
  default = null

• magic-nix-cache-action/README.md

  Line 117 in cec65ff

  | `use-flakehub` | Whether to upload build results to FlakeHub Cache (private beta). | | true |

  
  default = true