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                                                           |