merge translates into "合并" better (#11659)

* merge translates into "合并" better

* Update index.md
Co-authored-by: NIvan Blinkov <github@blinkov.ru>

docs/zh/engines/table-engines/mergetree-family/index.md

 ---
-machine_translated: true
-machine_translated_rev: 72537a2d527c63c07aa5d2361a8829f3895cf2bd
-toc_folder_title: "\u6885\u6811\u5BB6\u65CF"
+toc_folder_title: "合并树家族"
 toc_priority: 28
 ---