Fix nit

Reviewed-by: Matt Caswell <[email protected]>
Reviewed-by: Tomas Mraz <[email protected]>
(Merged from openssl#21795)

doc/designs/quic-design/server/quic-polling.md

@@ -125,7 +125,7 @@ contain a large number of design issues:
 
 - There are separate `EPOLL_CTL_ADD` and `EPOLL_CTL_MOD` calls which are needed
   to add a new FD registration and modify an existing FD registration, when
-  most of the time what is desired is an “upsert” (update or inser) call. Thus
+  most of the time what is desired is an “upsert” (update or insert) call. Thus
   callers have to track whether an FD has already been added or not.
 
 - Only one FD can be registered, modified, or unregistered per syscall, rather