docs: update the sdbus++ python installation

Update the sdbus++ python installation documentation, as it has changed
from setup.py to pyproject.toml.

Fixes: ed24aa516d98 ("sdbus++: convert from setup.py to pyproject.tom")
Change-Id: Ie50f99e2678777dde5b40e131d48b5017c192a20
Signed-off-by: Haiyue Wang <haiyuewa@163.com>
diff --git a/README.md b/README.md
index 4802467..c66ee5c 100644
--- a/README.md
+++ b/README.md
@@ -34,7 +34,7 @@
 
 ```sh
 cd tools
-./setup.py install
+pip install .
 ```
 
 ## C++ library