Fix typo for LTM_CFLAGS

Author: mkj

SMALL

@@ -50,7 +50,7 @@ Of course using small C libraries such as uClibc and dietlibc can also help.
 ---
 
 Libtommath has its own default CFLAGS to improve speed. You can use
-./configure LTM_CFLAGS=-O2
+./configure LTM_CFLAGS=-Os
 to reduce size at the expense of speed.
 
 If you have any queries, mail me and I'll see if I can help.