Fix regular expression in Arc Player (LGTM)

LGTM report:

    The escape sequence '\.' is equivalent to just '.', so the sequence may
    still represent a meta-character when it is used in a regular expression.

Signed-off-by: Stefan Weil <sw@weilnetz.de>

bigbluebutton-html5/imports/ui/components/external-video-player/custom-players/arc-player.jsx

 import loadScript from 'load-script';
 import React, { Component } from 'react'
 
-const MATCH_URL = new RegExp("https?:\/\/(\\w+)\.(instructuremedia.com)(\/embed)?\/([-abcdef0-9]+)");
+const MATCH_URL = new RegExp("https?:\/\/(\\w+)[.](instructuremedia.com)(\/embed)?\/([-abcdef0-9]+)");
 
 const SDK_URL = 'https://files.instructuremedia.com/instructure-media-script/instructure-media-1.1.0.js';