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.

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

src/bin/initdb/initdb.c

@@ -3350,7 +3350,7 @@ main(int argc, char *argv[])
 	check_need_password(authmethodlocal, authmethodhost);
 
 	if (!IsValidWalSegSize(wal_segment_size_mb * 1024 * 1024))
-		pg_fatal("argument of %s must be a power of 2 between 1 and 1024", "--wal-segsize");
+		pg_fatal("argument of %s must be a power of two between 1 and 1024", "--wal-segsize");
 
 	get_restricted_token();
 

src/bin/pg_resetwal/pg_resetwal.c

@@ -298,7 +298,7 @@ main(int argc, char *argv[])
 						exit(1);
 					set_wal_segsize = wal_segsize_mb * 1024 * 1024;
 					if (!IsValidWalSegSize(set_wal_segsize))
-						pg_fatal("argument of %s must be a power of 2 between 1 and 1024", "--wal-segsize");
+						pg_fatal("argument of %s must be a power of two between 1 and 1024", "--wal-segsize");
 					break;
 				}