Reword description and change NUL to null

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/include/mbedtls/oid.h b/include/mbedtls/oid.h
index 68ae723..7fa0141 100644
--- a/include/mbedtls/oid.h
+++ b/include/mbedtls/oid.h
@@ -479,9 +479,8 @@
  *                  heap. It must be freed by the caller using mbedtls_free().
  *
  * \param oid       #mbedtls_asn1_buf to populate with the DER-encoded OID
- * \param oid_str   string representation of the OID to parse, not
- *                  NUL-terminated
- * \param size      length of the OID string, not including any NUL terminator
+ * \param oid_str   string representation of the OID to parse
+ * \param size      length of the OID string, not including any null terminator
  *
  * \return          0 if successful
  * \return          #MBEDTLS_ERR_ASN1_INVALID_DATA if \p oid_str does not