Correct misleading debugging output

Usually, debug messages beginning with "=> and "<="
match up and indicate entering of and returning from
functions, respectively. This commit fixes one exception
to this rule in mbedtls_ssl_read_record(), which sometimes
printed two messages of the form "<= XXX".
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 23b066c..910e584 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -4322,7 +4322,7 @@
     }
     else
     {
-        MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= reuse previously read message" ) );
+        MBEDTLS_SSL_DEBUG_MSG( 2, ( "reuse previously read message" ) );
         ssl->keep_current_message = 0;
     }