docs: contributing: use '.' rather than 'source' to align with 'installation'

See commit 855ba1f07aeb437483138d48d7929f0ff8c38b67 .

docs/contributing.rst

@@ -34,7 +34,7 b' bash shell::'
         hg clone https://kallithea-scm.org/repos/kallithea
         cd kallithea
         python3 -m venv ../kallithea-venv
-        source ../kallithea-venv/bin/activate
+        . ../kallithea-venv/bin/activate
         pip install --upgrade pip setuptools
         pip install --upgrade -e . -r dev_requirements.txt python-ldap python-pam
         kallithea-cli config-create my.ini