Update _posts/2020-04-04-how-to-contribute-by-cpython-unittest.markdown

Co-Authored-By: Noah <[email protected]>

Author: youknowone

_posts/2020-04-04-how-to-contribute-by-cpython-unittest.markdown

@@ -19,7 +19,7 @@ Let's find an incompatibility issue and fix it.
 Here is a quick tip to run single unittest file.
 
 ```sh
-$ RUSTPYTHONLIB=Lib cargo run --release Lib/test/test_unicode.py
+$ RUSTPYTHONPATH=Lib cargo run --release Lib/test/test_unicode.py
 ```
 
 ## Add a new unittest file