Fix typos found by codespell

Author: DimitriPapadopoulos

doc/instop.html

@@ -217,9 +217,9 @@ <h2>
 </a>
 </h2>
 
-	This program is avaiable from <a href="https://freeradius.org/sub_projects/">.
+	This program is available from <a href="https://freeradius.org/sub_projects/">.
 	<br>
-	Download the version with the largest version number, older version are
+	Download the version with the largest version number, older versions are
 	only kept for reference.