Update uv installation guide

Author: squidfunk

docs/get-started.md

@@ -90,23 +90,20 @@ install Zensical with `uv`, use:
 
     ```
     uv init
-    source .venv/bin/activate
     uv add zensical
     ```
 
 === ":fontawesome-brands-windows: Windows"
 
     ```
     uv init
-    .venv\Scripts\activate
     uv add zensical
     ```
 
 === ":material-linux: Linux"
 
     ```
     uv init
-    source .venv/bin/activate
     uv add zensical
     ```