Fix another typo where 'PSK' was 'PKS'

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/ssl_tls12_client.c b/library/ssl_tls12_client.c
index eda8891..7a17452 100644
--- a/library/ssl_tls12_client.c
+++ b/library/ssl_tls12_client.c
@@ -1971,7 +1971,7 @@
     }
 
     /*
-     * Note: we currently ignore the PKS identity hint, as we only allow one
+     * Note: we currently ignore the PSK identity hint, as we only allow one
      * PSK to be provisioned on the client. This could be changed later if
      * someone needs that feature.
      */