Fix syntax error in web_client.md
Fix syntax error in web_client.md

This fixes [#594](https://github.com/slackapi/node-slack-sdk/issues/594) syntax error for the web_client documentation.
slackhq/node-slack-client,slackapi/node-slack-sdk,slackapi/node-slack-sdk,slackapi/node-slack-sdk,slackhq/node-slack-sdk,slackapi/node-slack-sdk