Fix typos in doc (#21)

Author: gagbo

README.md

@@ -10,7 +10,7 @@ inspect usage metrics from your code.
 You can optionnally add alerting rules so that code annotations make Prometheus
 trigger alerts directly from production usage:
 
-![a Slack bot is posting an alert directly in the channel](../../assets/slack-alert-example.png)
+![a Slack bot is posting an alert directly in the channel](./assets/slack-alert-example.png)
 
 A fully working use-case and example of library usage is available in the
 [examples/web](./examples/web) subdirectory

examples/web/alertmanager.yml

@@ -1,6 +1,6 @@
 global:
-  # Also possible to place this URL in a file.
-  # Ex: `slack_api_url_file: '/etc/alertmanager/slack_url'`
+  # Also possible to use the URL directly
+  # Ex: `slack_api_url: 'https://slack.com/...'`
   slack_api_url_file: '/etc/alertmanager/slack_url'
 
 route: