Increase line height of markdown text

Summary: Lines were a bit squashed together. Taken the value from workplace, as that's the style we've been copying so far.

Reviewed By: priteshrnandgaonkar

Differential Revision: D19430279

fbshipit-source-id: 52cc8167d6acaa6af42caaa67a693b5fda492701

src/ui/components/Markdown.tsx

@@ -19,6 +19,7 @@ const Container = styled.div({
 const Row = styled.div({
   marginTop: 5,
   marginBottom: 5,
+  lineHeight: 1.34,
 });
 const Heading = styled.div<{level: number}>(props => ({
   fontSize: props.level === 1 ? 18 : 12,