Fix HTTP method of getCurrentActivity (#3296)

closes #3295

## Proposed changes

Fix method for `getCurrentActivity`.

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- [x] Bugfix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

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### Reviewers: @webdriverio/technical-committee

packages/webdriver/protocol/appium.json

@@ -270,7 +270,7 @@
     }
   },
   "/session/:sessionId/appium/device/current_activity": {
-    "POST": {
+    "GET": {
       "command": "getCurrentActivity",
       "description": "Get the name of the current Android activity.",
       "ref": "http://appium.io/docs/en/commands/device/activity/current-activity/",