pkg(8): Remove a double word in a source code comment

- s/that that/that/

MFC after:	3 days

usr.sbin/pkg/pkg.c

@@ -1233,7 +1233,7 @@ main(int argc, char *argv[])
 		/*
 		 * To allow 'pkg -N' to be used as a reliable test for whether
 		 * a system is configured to use pkg, don't bootstrap pkg
-		 * when that that option is passed.
+		 * when that option is passed.
 		 */
 		if (activation_test)
 			errx(EXIT_FAILURE, "pkg is not installed");