Wording improvement

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/include/mbedtls/ccm.h b/include/mbedtls/ccm.h
index c4e310d..b8964d8 100644
--- a/include/mbedtls/ccm.h
+++ b/include/mbedtls/ccm.h
@@ -375,7 +375,7 @@
  *                  ways:
  *                  - Immediate output: the output length is always equal
  *                    to the input length.
- *                  - Buffered output: but for the last part of input data,
+ *                  - Buffered output: except for the last part of input data,
  *                    the output consists of a whole number of 16-byte blocks.
  *                    If the total input length so far (not including
  *                    associated data) is 16 \* *B* + *A* with *A* < 16 then