docs: modify contributing instruction + install pytest-cov in github action

Author: kayjan

.github/workflows/pytest.yml

@@ -44,5 +44,5 @@ jobs:
           isort --profile black . --check
       - name: Test with pytest
         run: |
-          python -m pip install pytest pytest-benchmark coverage
+          python -m pip install pytest pytest-cov pytest-benchmark coverage
           pytest --cov-report=term-missing --cov-config=pyproject.toml --cov=bigtree

docs/home/contributing.md

@@ -77,7 +77,7 @@ Run the following lines of code and ensure the <mark>unit tests pass</mark> and
 <mark>code coverage is 100%</mark>.
 
 ```console
-$ python -m pip install pytest coverage
+$ python -m pip install pytest pytest-cov coverage
 $ pytest --cov-report=term-missing --cov-config=pyproject.toml --cov=bigtree
 ```