Revise push-example in remotes section

The new example uses a fictive feature-branch to don't introduce the
"bad" (i.e. we don't want to encourage it)  habit of pushing to main.

Co-authored-by: Tobias Koelling <tobias.koelling@mpimet.mpg.de>

lectures/git/slides.qmd

@@ -320,7 +320,7 @@ Example from [kernel.org](https://git.kernel.org/pub/scm/linux/kernel/git/torval
   ```
 * Push your local references to the remote repo
   ```bash
-  git push origin main
+  git push origin feature-branch
   ```
 * Pull remote changes to your local repo
   ```bash