Update tutorial.html

I was confused by this for a minute until I realized it meant .foo and not some magic member.

Please correct me if I'm wrong though 😄

Author: nagstaku

doc/learning/tutorial.html

@@ -122,7 +122,7 @@ <h2 id="references">References</h2>
         <li><code>self</code> refers to the current object.</li>
         <li><code>$</code> refers to the outer-most object.</li>
         <li><code>['foo']</code> looks up a field.</li>
-        <li><code>.f</code> can be used if the field name is an identifier.</li>
+        <li><code>.foo</code> can be used if the field name is an identifier.</li>
         <li><code>[10]</code> looks up an array element.</li>
         <li>Arbitrarily long paths are allowed.</li>
         <li>Array slices like <code>arr[10:20:2]</code> are allowed, like in Python.</li>