Fix coding style

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>

Co-authored-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 670e761..22c884c 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -6560,7 +6560,7 @@
             const mbedtls_ecp_keypair *ec = mbedtls_pk_ec( *pk );
             if( ec == NULL )
             {
-                MBEDTLS_SSL_DEBUG_MSG( 1, ( "mbedtls_pk_ec() returned MULL"));
+                MBEDTLS_SSL_DEBUG_MSG( 1, ( "mbedtls_pk_ec() returned NULL" ) );
                 return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
             }