Update global-context.md

[IllegalCreed]

问题描述:

原文是: When the useSlideContext composable is used in a file, the automatic injection of $slidev will be disabled. You need to manually get the $slidev object to the useSlideContext function.

首先我英语不太好，但我确定这里想表达的意思应该不是把 $slidev 当参数传递给 useSlideContext。而是从 useSlideContext 获取 $slidev。所以这个原文是不是也需要修改成 get ... from 更好。虽然 get ... to 也能理解。