Fix doxygen for MBEDTLS_SSL_KEYING_MATERIAL_EXPORT

Error was introduced while resolving a merge conflict.

Signed-off-by: Max Fillinger <maximilian.fillinger@foxcrypto.com>
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index 40e16e1..d5a4883 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -737,7 +737,7 @@
  */
 //#define MBEDTLS_SSL_RECORD_SIZE_LIMIT
 
-/*
+/**
  * \def MBEDTLS_SSL_KEYING_MATERIAL_EXPORT
  *
  * When this option is enabled, the client and server can extract additional