Test coverage c extension doc fix

Currently in the test coverage section, specifically 5.2.1.4, it mentions building the C extension for coverage.py. For a while now coverage.py has come with the C extension by default, so this step is almost never required (see here: https://coverage.readthedocs.io/en/coverage-5.1/install.html#c-extension).
I'm thinking it would be worth mentioning in the guide that installing the C extension is only required if 'coverage --version' says 'Without C extension'.

從測試涵蓋率部分開始，具體查看小節 5.2.1.4，並將其中關於 C 擴充功能的說明與連結的 coverage.py 文件進行比較。當指南說明只有在 coverage --version 回報 “Without C extension” 時才需要安裝該擴充功能，即表示完成。