[git/zh-cn] Add periods for better reading in the multiple lines.

[linglilongyi]

• I solemnly swear that this is all original content of which I am the original author
• Pull request title is prepended with [language/lang-code] (example [python/fr] for Python in French or [java] for multiple Java translations)
• Pull request touches only one file (or a set of logically related files with similar changes made)
• Content changes are aimed at intermediate to experienced programmers (this is a poor format for explaining fundamental programming concepts)
• If you've changed any part of the YAML Frontmatter, make sure it is formatted according to CONTRIBUTING.md

In the previous version, the multiple lines in markdown will compile into a paragraph as expected, but sentences has been phrasing wrong:

一个 git 提交就是一组更改或者对工作目录操作的快照 比如你添加了5个文件，删除了2个文件，那么这些变化就会被写入一个提交比如你添加了5个文件，删除了2个文件，那么这些变化就会被写入一个提交中 而这个提交之后也可以被决定是否推送到另一个版本库中

It should be:

一个 git 提交就是一组更改或者对工作目录操作的快照 比如你添加了5个文件，删除了2个文件，那么这些变化就会被写入一个提交。比如你添加了5个文件，删除了2个文件，那么这些变化就会被写入一个提交中。而这个提交之后也可以被决定是否推送到另一个版本库中