Fix typos in C header files

Signed-off-by: Tobias Nießen <tniessen@tnie.de>
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index fff4240..b1d67f7 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -453,7 +453,7 @@
  *       be overridden, but the wrapper functions mbedtls_aes_decrypt and mbedtls_aes_encrypt
  *       must stay untouched.
  *
- * \note If you use the AES_xxx_ALT macros, then is is recommended to also set
+ * \note If you use the AES_xxx_ALT macros, then it is recommended to also set
  *       MBEDTLS_AES_ROM_TABLES in order to help the linker garbage-collect the AES
  *       tables.
  *