Change z-index of TOC to fix cell overlapping (#157)

src/TableOfContents.jl

@@ -147,7 +147,7 @@ const toc_css = """
 		/* That is, viewport minus top minus Live Docs */
 		max-height: calc(100vh - 5rem - 56px);
 		overflow: auto;
-		z-index: 10;
+		z-index: 20;
 		background: white;
 	}
 }