docs: improve throws examples (#1113)

When throws is invoked with two arguments, where the second is a string or regexp, the second argument is the **errMsgMatcher**. The current examples imply that this string is the **message**  rather than the matcher.

lib/chai/interface/assert.js

@@ -1940,8 +1940,8 @@ module.exports = function (chai, util) {
    * If `errMsgMatcher` is provided, it also asserts that the error thrown will have a
    * message matching `errMsgMatcher`.
    *
-   *     assert.throws(fn, 'function throws a reference error');
-   *     assert.throws(fn, /function throws a reference error/);
+   *     assert.throws(fn, 'Error thrown must have this msg');
+   *     assert.throws(fn, /Error thrown must have a msg that matches this/);
    *     assert.throws(fn, ReferenceError);
    *     assert.throws(fn, errorInstance);
    *     assert.throws(fn, ReferenceError, 'Error thrown must be a ReferenceError and have this msg');