Add newline at end of proof.css file

Appended a newline character to the end of sphinx_proof/_static/proof.css to follow POSIX file standards and improve compatibility with some tools.

Author: douden

sphinx_proof/_static/proof.css

@@ -256,4 +256,4 @@ div.notation {
 
 div.notation p.admonition-title {
 	background-color: var(--notation-title-color);
-}
+}