doc: use correct signature for assert()

The message argument is optional for both assert() and
assert.ok(). This commit makes message optional for assert().

PR-URL: https://github.com/joyent/node/pull/9003
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

doc/api/assert.markdown

@@ -9,7 +9,7 @@ access it with `require('assert')`.
 
 Throws an exception that displays the values for `actual` and `expected` separated by the provided operator.
 
-## assert(value, message), assert.ok(value[, message])
+## assert(value[, message]), assert.ok(value[, message])
 
 Tests if value is truthy, it is equivalent to `assert.equal(true, !!value, message);`