Fix minor error message style violation.

Primary error messages should not end with a period, since they're
generally not written as full sentences.  Oversight in 41493bac3.

src/backend/libpq/auth.c

@@ -716,7 +716,7 @@ CheckMD5Auth(Port *port, char **logdetail)
 	if (!pg_backend_random(md5Salt, 4))
 	{
 		ereport(LOG,
-				(errmsg("could not generate random MD5 salt.")));
+				(errmsg("could not generate random MD5 salt")));
 		return STATUS_ERROR;
 	}