index.html: fixed the incorrect URL to the flight review documentation

app/plot_app/templates/index.html

@@ -7,7 +7,7 @@
 {% if is_plot_page %}
 <div class="alert alert-primary"> <!-- alternatives: alert-info, alert-primary, alert-secondary -->
   Do you need help with interpreting the plots?
-  See <a href="https://docs.px4.io/en/log/flight_review.html" target="_blank" class="alert-link">here</a>.
+  See <a href="https://docs.px4.io/main/en/log/flight_review.html" target="_blank" class="alert-link">here</a>.
 </div>
 {% endif %}