Update configs README

This commit updates configs/ext/README.md to reflect the necessary files
which were removed from Mbed TLS.

Signed-off-by: Harry Ramsey <harry.ramsey@arm.com>
diff --git a/configs/crypto-config-suite-b.h b/configs/crypto-config-suite-b.h
index 3eea061..3fec3d0 100644
--- a/configs/crypto-config-suite-b.h
+++ b/configs/crypto-config-suite-b.h
@@ -1,5 +1,5 @@
 /**
- * \file crypto-config-symmetric-only.h
+ * \file crypto-config-suite-b.h
  *
  * \brief \brief Minimal crypto configuration for
  * TLS NSA Suite B Profile (RFC 6460).