nrf_modem: doc: Change GNSS interface name

Changed GNSS interface name in documentation from
"Global Satellite Navigation System" to "GNSS". The name of the
interface in modemlib is "GNSS" and in general all documentation
refers to "GNSS interface" or "GNSS API".

Signed-off-by: Tommi Kangas <tommi.kangas@nordicsemi.no>

Author: tokangas

nrf_modem/doc/gnss_interface.rst

@@ -1,8 +1,8 @@
 .. _nrf_modem_gnss:
 .. _gnss_interface:
 
-Global Navigation Satellite System
-##################################
+GNSS
+####
 
 .. contents::
    :local: