Fix typo

Author: DelazJ

python/user.py

@@ -104,7 +104,7 @@ def my_sum(value1, value2, feature, parent):
     <h2>Example usage:</h2>
     <ul>
       <li>my_sum(5, 8) -> 13</li>
-      <li>my_sum(\"fiel1\", \"field2\") -> 42</li>
+      <li>my_sum(\"field1\", \"field2\") -> 42</li>
     </ul>
     \"\"\"
     return value1 + value2