DOC: fix typo in Prometheus exporter doc
It's just a minor typo in a section title.
diff --git a/contrib/prometheus-exporter/README b/contrib/prometheus-exporter/README
index 1d1ec0a..d33e194 100644
--- a/contrib/prometheus-exporter/README
+++ b/contrib/prometheus-exporter/README
@@ -148,7 +148,7 @@
| haproxy_frontend_http_comp_responses_total | Total number of HTTP responses that were compressed. |
+-------------------------------------------------+------------------------------------------------------------------------------+
-* Bakcned metrics
+* Backend metrics
+-----------------------------------------------------+--------------------------------------------------------------------------+
| Metric name | Description |