Revise Changelog to clarify and add credit
diff --git a/ChangeLog b/ChangeLog
index 865ea61..a217fa6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -15,11 +15,8 @@
    * Fixed the sample applications gen_key.c, cert_req.c and cert_write.c for
      builds where the configuration MBEDTLS_PEM_WRITE_C is not defined. Found
      by inestlerode. #559.
-   * Fixed default threading implementation to avoid accidental double
-     initialisations and double frees.
-   * Fixed the sample applications gen_key.c, cert_req.c and cert_write.c for
-     builds where the configuration MBEDTLS_PEM_WRITE_C is not defined. Found
-     by inestlerode. #559.
+   * Fixed pthread implementation to avoid unintended double initialisations
+     and double frees. (found by Niklas Amnebratt)
 
 = mbed TLS 2.1.5 branch released 2016-06-28