Adding let when declaring a variable so it's consistent with other ex…

…amples (#2153)

files/en-us/learn/javascript/first_steps/strings/index.html

@@ -87,7 +87,7 @@ <h3 id="Escaping_characters_in_a_string">Escaping characters in a string</h3>
 
 <p>To fix our previous problem code line, we need to escape the problem quote mark. Escaping characters means that we do something to them to make sure they are recognized as text, not part of the code. In JavaScript, we do this by putting a backslash just before the character. Try this:</p>
 
-<pre class="brush: js">bigmouth = 'I\'ve got no right to take my place...';
+<pre class="brush: js">let bigmouth = 'I\'ve got no right to take my place...';
 bigmouth;</pre>
 
 <p>This works fine. You can escape other characters in the same way, e.g. <code>\"</code>,  and there are some special codes besides. See <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/String#escape_notation">Escape notation</a> for more details.</p>