docs(README): use raw GitHub URL for logo (PyPI compatibility)

why: PyPI renders external image URLs but the docs site URL may not
be whitelisted. Raw GitHub URLs are the canonical pattern for
images that work on both GitHub and PyPI.

what:
- Change logo src from libtmux.git-pull.com to raw.githubusercontent.com

Author: tony

README.md

@@ -2,7 +2,7 @@
   <h1>⚙️ libtmux</h1>
   <p><strong>Drive tmux from Python: typed, object-oriented control over servers, sessions, windows, and panes.</strong></p>
   <p>
-    <a href="https://libtmux.git-pull.com/"><img src="https://libtmux.git-pull.com/_static/img/libtmux.svg" alt="libtmux logo" height="120"></a>
+    <a href="https://libtmux.git-pull.com/"><img src="https://raw.githubusercontent.com/tmux-python/libtmux/master/docs/_static/img/libtmux.svg" alt="libtmux logo" height="120"></a>
   </p>
   <p>
     <a href="https://pypi.org/project/libtmux/"><img src="https://img.shields.io/pypi/v/libtmux.svg" alt="PyPI version"></a>