[Docs] Change docs to reflect DevMode is enabled.
[Docs] Change docs to reflect DevMode is enabled.

Changed the howToDevMode doc to reflect that devMode is
enabled by default on target devices.

Resolves: LE-5926
Change-Id: Iab11c7ce3b65511b04e2c63501a51fb2915cb59e

legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af