pkwrite.c: write ChangeLog accurately

The heap memory is used for both RSA and EC keys. So removing `RSA`
in the ChangeLog.

Signed-off-by: Yanray Wang <yanray.wang@arm.com>
diff --git a/ChangeLog.d/pkwrite-pem-use-heap.txt b/ChangeLog.d/pkwrite-pem-use-heap.txt
index d2e7129..11db7b6 100644
--- a/ChangeLog.d/pkwrite-pem-use-heap.txt
+++ b/ChangeLog.d/pkwrite-pem-use-heap.txt
@@ -1,4 +1,4 @@
 Changes
-   * Use heap memory to allocate DER encoded RSA public/private key.
+   * Use heap memory to allocate DER encoded public/private key.
      This reduces stack usage significantly for writing a public/private
      key to a PEM string.