Minor fixes and improvements in TLS 1.3 key schedule documentation

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 188b947..7b78c73 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -379,7 +379,7 @@
                                      const unsigned char *random, size_t rlen,
                                      unsigned char *dstbuf, size_t dlen );
 
-/* cipher.h exports the maximum IV, key and block length from all
+/* cipher.h exports the maximum IV, key and block length from
  * all ciphers enabled in the config, regardless of whether those
  * ciphers are actually usable in SSL/TLS. Notably, XTS is enabled
  * in the default configuration and uses 64 Byte keys, but it is