doc: Update 'guix import json' example.

This is a followup to d78010b8.

* doc/guix.texi (Invoking guix import): Change 'guix import json'
example to refer to "gettext", not "gcc".

doc/guix.texi

@@ -8674,7 +8674,7 @@ example package definition in JSON format:
   "synopsis": "Hello, GNU world: An example GNU package",
   "description": "GNU Hello prints a greeting.",
   "license": "GPL-3.0+",
-  "native-inputs": ["gcc@@6"]
+  "native-inputs": ["gettext"]
 @}
 @end example