docs: use log.total_logs in event export example

Replace hardcoded '1' with log.total_logs in the Event Export Example to
correctly handle multi-log BBL files, making it consistent with the
Complete Export Example.

Author: nerdCopter

CRATE_USAGE.md

@@ -167,7 +167,7 @@ fn main() -> anyhow::Result<()> {
         export_to_event(
             Path::new("flight.BBL"),
             0,  // log index
-            1,  // total_logs
+            log.total_logs,
             &log.event_frames,
             &export_opts
         )?;