doc: update title for consistency

Other NFC samples start with "NFC:". Update to be consistent.

Signed-off-by: Ruth Fuchss <[email protected]>

Author: Ruth Fuchss

samples/nfc/system_off/README.rst

@@ -1,7 +1,7 @@
 .. _nrf-nfc-system-off-sample:
 
-NFC System OFF
-##############
+NFC: System OFF
+###############
 
 The NFC System OFF sample shows how to make the NFC Tag device wake up from the System OFF mode when it detects the NFC field.
 The sample uses the :ref:`nfc`.