Reword user-facing message for "power of two"

While there are numerous instances of using "power of 2" in the code,
translated user-facing messages use "power of two". Fix two instances
which used "power of 2" instead.

This is a backpatch of 95fff2abee in master.

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20230829.175615.682972421946735863.horikyota.ntt@gmail.com
Backpatch-through: v16

src/bin/initdb/initdb.c

@@ -3356,7 +3356,7 @@ main(int argc, char *argv[])
 		if (endptr == str_wal_segment_size_mb || *endptr != '\0')
 			pg_fatal("argument of --wal-segsize must be a number");
 		if (!IsValidWalSegSize(wal_segment_size_mb * 1024 * 1024))
-			pg_fatal("argument of --wal-segsize must be a power of 2 between 1 and 1024");
+			pg_fatal("argument of --wal-segsize must be a power of two between 1 and 1024");
 	}
 
 	get_restricted_token();

src/bin/pg_resetwal/pg_resetwal.c

@@ -295,7 +295,7 @@ main(int argc, char *argv[])
 				if (endptr == optarg || *endptr != '\0' || errno != 0)
 					pg_fatal("argument of --wal-segsize must be a number");
 				if (!IsValidWalSegSize(set_wal_segsize))
-					pg_fatal("argument of --wal-segsize must be a power of 2 between 1 and 1024");
+					pg_fatal("argument of --wal-segsize must be a power of two between 1 and 1024");
 				break;
 
 			default: