Update GitHub issue templates

Author: seratch

.github/ISSUE_TEMPLATE/02_enhancement.md

@@ -10,10 +10,11 @@ assignees: ""
 
 ### Category (place an `x` in each of the `[ ]`)
 
-- [ ] **slack_sdk.web.WebClient** (Web API client)
-- [ ] **slack_sdk.webhook.WebhookClient** (Incoming Webhook, response_url sender)
+- [ ] **slack_sdk.web.WebClient (sync/async)** (Web API client)
+- [ ] **slack_sdk.webhook.WebhookClient (sync/async)** (Incoming Webhook, response_url sender)
 - [ ] **slack_sdk.models** (UI component builders)
 - [ ] **slack_sdk.oauth** (OAuth Flow Utilities)
+- [ ] **slack_sdk.socket_mode** (Socket Mode client)
 - [ ] **slack_sdk.rtm.RTMClient** (RTM client)
 - [ ] **slack_sdk.signature** (Request Signature Verifier)