Fix carelessly copy pasted comment

Signed-off-by: Paul Elliott <[email protected]>

library/base64.c

@@ -109,7 +109,7 @@ static void mbedtls_base64_cond_assign_uchar(unsigned char * dest, const unsigne
 #pragma warning( disable : 4146 )
 #endif
 
-    /* Generate bitmask from condition, mask will either be 0xFFFFFFFF or 0 */
+    /* Generate bitmask from condition, mask will either be 0xFF or 0 */
     unsigned char mask = ( condition | -condition );
     mask >>= 7;
     mask = -mask;