Docs: Center the README logo

The <picture> sat outside any centered block, so GitHub drew the logo at
the left edge. Wrap it in <p align="center"> like the badges below it
(GitHub strips style attributes but keeps align). Also drop the alt on
<source>, which ignores it, and give the <img> a real description.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: r41k0u

README.md

@@ -1,14 +1,15 @@
-<picture>
-  <source
-    media="(prefers-color-scheme: light)"
-    srcset="https://github.com/user-attachments/assets/f3738131-d7cb-4b5c-8699-c7010295a159"
-    width="450"
-    alt="Light‐mode image">
-  <img
-    src="https://github.com/user-attachments/assets/b175bf39-23cb-475d-a6e1-7b5c99a1ed72"
-    width="450"
-    alt="Dark‐mode image">
-</picture>
+<p align="center">
+  <picture>
+    <source
+      media="(prefers-color-scheme: light)"
+      srcset="https://github.com/user-attachments/assets/f3738131-d7cb-4b5c-8699-c7010295a159"
+      width="450">
+    <img
+      src="https://github.com/user-attachments/assets/b175bf39-23cb-475d-a6e1-7b5c99a1ed72"
+      width="450"
+      alt="PythonBPF logo">
+  </picture>
+</p>
 <!-- Badges -->
 <p align="center">
   <!-- PyPI -->