Update _language-zh.yml

This commit improves the Chinese localization of `environment-proof-title`.

Author: sun123zxy

src/resources/language/_language-zh.yml

@@ -98,7 +98,7 @@ crossref-lof-title: "图索引"
 crossref-lot-title: "表索引"
 crossref-lol-title: "列表索引"
 
-environment-proof-title: "证"
+environment-proof-title: "证明"
 environment-remark-title: "注记"
 environment-solution-title: "解"