[apm] fix typo in template.
diff --git a/templates/datadog.conf.erb b/templates/datadog.conf.erb
index dfb26c4..d53da7f 100644
--- a/templates/datadog.conf.erb
+++ b/templates/datadog.conf.erb
@@ -417,5 +417,5 @@
 
 <% if not @apm_env.empty? -%>
 [trace.agent]
-env = <%= @apm_env %>
+env: <%= @apm_env %>
 <% end -%>