Touch up a sentence

Author: yegeunyang

src/doc/rustc-dev-guide/src/getting-started.md

@@ -139,7 +139,7 @@ pull request, continuing the work on the feature.
 
 ### Writing tests
 
-Issues that have been resolved but do not have a regression test are marked with the `E-needs-test` label. Writing unit tests is a low-risk, lower-priority task that offers new contributors a great opportunity to familiarize themselves with the codebase.
+Issues that have been resolved but do not have a regression test are marked with the `E-needs-test` label. Writing unit tests is a low-risk, lower-priority task that offers new contributors a great opportunity to familiarize themselves with the testing infrastructure and contribution workflow.
 
 ### Contributing to std (standard library)