change domain.com to example.com

IMO it should be avoided to use domains, which are owned by other projects/persons/organizations...

example.com is one of the domains reserved for documentation purposes

See
https://example.com/
https://www.iana.org/domains/reserved
for more information

src/components/HomePage.js

@@ -398,7 +398,7 @@ export default React.createClass({
             prompt = [
                 <div key="inputBox" className="mxt_HomePage_inputBox">
                     <form onSubmit={ this.onSubmit }>
-                        <input autoFocus className="mxt_HomePage_inputBox_prompt" value={ this.state.entity } ref="prompt" size="36" type="text" placeholder="#room:domain.com, @user:domain.com or +group:domain.com" />
+                        <input autoFocus className="mxt_HomePage_inputBox_prompt" value={ this.state.entity } ref="prompt" size="36" type="text" placeholder="#room:example.com, @user:example.com or +group:example.com" />
                         <input className="mxt_HomePage_inputBox_button" type="submit" value="Get link!" />
                     </form>
                     { error }