fix various issues

- wrong typo in comments
- replace psk null check with key_exchange_mode check
- set psk NULL when error return in export hs psk

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/programs/ssl/ssl_server2.c b/programs/ssl/ssl_server2.c
index d26fcc8..d0193e4 100644
--- a/programs/ssl/ssl_server2.c
+++ b/programs/ssl/ssl_server2.c
@@ -1860,9 +1860,9 @@
                 opt.tls13_kex_modes = MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_PSK_ALL;
             else if( strcmp( q, "all" ) == 0 )
                 opt.tls13_kex_modes = MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_ALL;
-            /* `psk_or_ephemeral` exists in theory and is not recommend in practise.
+            /* `psk_or_ephemeral` exists in theory and is not recommended in practice.
              * In server side, if needed extensions are received, psk or ephemeral
-             * mode will be set. Add this mode only for test purpose to improve
+             * mode will be set. Add this mode only for test purposes to improve
              * test coverage.
              */
             else if( strcmp( q, "psk_or_ephemeral" ) == 0 )