Improve alt text of flag diagram

Co-authored-by: Alistair Lynn <[email protected]>

src/index.html

@@ -172,7 +172,7 @@ <h3><a class="anchor" id="arena-specification"></a>Arena</h3>
           SRO reserves the right to have match officials in the arena during games.
       </ol>
       <h3><a class="anchor" id="flag-specification"></a>Robot flags</h3>
-      <img src="./fig-flag.svg" alt="Robot flag" width="40%" height="auto">
+      <img src="./fig-flag.svg" alt="A small red flag, attached to a flag pole" width="40%" height="auto">
       <ol>
         <li>
           A "robot flag" is a removable identifier that will be attached to a robot throughout a match. It allows spectators to easily associate a robot with its starting area.