api docs: link to html version of api-trace2

In f6d25d7 (api docs: document that BUG() emits a trace2 error event,
2021-04-13), a link to the plain text version of api-trace2 was added in
`technical/api-error-handling.txt`.

All of our other `link:`s point to the html versions.  Do the same here.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-error-handling.txt

@@ -46,7 +46,7 @@ parse-options.c.
   returns -1 after reporting the situation to the caller.
 
 These reports will be logged via the trace2 facility. See the "error"
-event in link:api-trace2.txt[trace2 API].
+event in link:api-trace2.html[trace2 API].
 
 Customizable error handlers
 ---------------------------