[plugins] change pip venv shell hook recommendation to be posix-compatible (#1939)

## Summary

This was requested on discord.

The `source` is non-posix. Using the period (`.`) is posix.

## How was it tested?

did not test

Author: savil

plugins/pip/venvShellHook.sh

@@ -5,4 +5,4 @@ if ! [ -d "$VENV_DIR" ]; then
     python3 -m venv "$VENV_DIR"
 fi
 
-echo "You can activate the virtual environment by running 'source \$VENV_DIR/bin/activate'" >&2
+echo "You can activate the virtual environment by running '. \$VENV_DIR/bin/activate' (for fish shell, replace '.' with 'source')" >&2