docs: Update year in change log (#20670)

* Update year in CHANGELOG.zh-CN.md

Update year in change log of 3.26.6 from 2019 to 2020

* Update year in CHANGELOG.en-US.md

Update year in change log of 3.26.6 from 2019 to 2020

CHANGELOG.en-US.md

@@ -17,7 +17,7 @@ timeline: true
 
 ## 3.26.6
 
-`2019-01-03`
+`2020-01-03`
 
 - 📢 v3 branch changes from `master` to `3.x-stable` and v4 merge into `master`.
 - 💄 Improve Steps `type="navigation"` UI in small screen. [#20545](https://github.com/ant-design/ant-design/pull/20545)

CHANGELOG.zh-CN.md

@@ -17,7 +17,7 @@ timeline: true
 
 ## 3.26.6
 
-`2019-01-03`
+`2020-01-03`
 
 - 📢 v3 分支由 `master` 进入 `3.x-stable`，v4 合入 `master`。
 - 💄 优化 Steps `type="navigation"` 在小屏幕下的样式。[#20545](https://github.com/ant-design/ant-design/pull/20545)