Fix typo

backend/templates/edit.html

@@ -30,7 +30,7 @@
                 <input type="text" id="css_filter" name="css_filter" value="{{watch.css_filter}}"
                        size="25" placeholder=".class-name or #some-id, or other CSS selector rule."/>
                 <span class="pure-form-message-inline">Limit text to this CSS rule, only text matching this CSS rule is included.<br/>
-                    Please be sure that you throughly understand how to write CSS selector rules before filing an issue on GitHub!
+                    Please be sure that you thoroughly understand how to write CSS selector rules before filing an issue on GitHub!
                 </span>
             </div>
             <br/>