Fix tags style

Using `flex` works much better because we don't have to account for the
margin of the following element being shortened.

go/blog/blog.go

var defaultStyle = `
article {
	margin-bottom: 2em;
	clear: left;
	margin-bottom: 1em;
}
article header {
}
.tags {
	display: flex;
	list-style-type: none;
	padding: 0;
}
.tags .tag-link {
	float: left;
	color: black;
	margin-right: 0.5em;
}
	color: #555;
	font-size: smaller;
	margin-bottom: 1em;
	clear: left;
}
`