Fix virtual env activation instruction in README.md (#34)

Replace `venv/` with `.venv/` in the virtual environment
activation instruction for Unix/macOS, as `.venv` is used as the
virtual environment name for these operating systems in the
previous virtual environment creation step.

Author: pavelacamposp

README.md

@@ -93,7 +93,7 @@ Follow these steps to create a virtual environment and install this package:
 4. Activate the virtual environment:
     - Unix/macOS:
         ```bash
-        source venv/bin/activate
+        source .venv/bin/activate
         ```
     - Windows:
         ```cmd