setup: change description (was that of sphinx-copybutton)

- The description metadata in setup.py is wrong, it seems to have not
  been changed from sphinx-copybutton.
- Noticed when searching on PyPI

Author: rkdarst

setup.py

@@ -16,7 +16,7 @@
 setup(
     name="sphinx-thebe",
     version=version,
-    description="Add a copy button to each of your code cells.",
+    description="Integrate interactive code blocks into your documentation with Thebelab and Binder.",
     long_description=readme_text,
     long_description_content_type="text/markdown",
     author="Executable Book Project",