[str.h] Fixing str_slice size calculation

src/str.h

@@ -142,7 +142,7 @@ str_slice(struct str str, size_t from, size_t to)
 	     + ( to <= str.size ) * to;
 	
 	str.data += from;
-	str.size = from - to;
+	str.size = to - from;
 
 	return str;