CLEANUP: contrib/prometheus-exporter: typo fixes for ssl reuse metric

A typo I identified while having a look to our metric inventory.
(s/frontent/frontend)
diff --git a/contrib/prometheus-exporter/service-prometheus.c b/contrib/prometheus-exporter/service-prometheus.c
index 952558c..009e817 100644
--- a/contrib/prometheus-exporter/service-prometheus.c
+++ b/contrib/prometheus-exporter/service-prometheus.c
@@ -485,7 +485,7 @@
 	[INF_MAX_SSL_RATE]                   = IST("max_ssl_rate"),
 	[INF_SSL_FRONTEND_KEY_RATE]          = IST("current_frontend_ssl_key_rate"),
 	[INF_SSL_FRONTEND_MAX_KEY_RATE]      = IST("max_frontend_ssl_key_rate"),
-	[INF_SSL_FRONTEND_SESSION_REUSE_PCT] = IST("frontent_ssl_reuse"),
+	[INF_SSL_FRONTEND_SESSION_REUSE_PCT] = IST("frontend_ssl_reuse"),
 	[INF_SSL_BACKEND_KEY_RATE]           = IST("current_backend_ssl_key_rate"),
 	[INF_SSL_BACKEND_MAX_KEY_RATE]       = IST("max_backend_ssl_key_rate"),
 	[INF_SSL_CACHE_LOOKUPS]              = IST("ssl_cache_lookups_total"),