doc: Documentation issue in 'connectivity/networking/api/tftp'

**Describe the bug**

Wrong Kconfig option is described for enabling the TFTP client library. It should be `CONFIG_TFTP_LIB` instead of `CONFIG_MQTT_SN_LIB`.

**Environment**

* Page: `connectivity/networking/api/tftp`
* Version: main
* SHA-1: b9453d52ed352a031b89b77ed8159846dbc1c70e
