MINOR: doc: fix busy-polling performance reference

busy-polling parameter was forgotten in the list of performance tuning

Signed-off-by: William Dauchy <w.dauchy@criteo.com>
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 73d694c..05ca6db 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -631,6 +631,7 @@
    - wurfl-cache-size
 
  * Performance tuning
+   - busy-polling
    - max-spread-checks
    - maxconn
    - maxconnrate