update the REPL intro text to make it clear where the expression goes

[gsouf]

Something small, but took me a some time before I realized that there is actually an input at the bottom of the page. I tried to click on the text Enter an expression to evaluate. Because it's grayed out it looked like the placeholder of an input field. At first I thought the REPL was broken. With a monitor in portrait mode (as per the screenshot below), the actual input is very far at the very bottom so I didn't notice it at first (also grayed out, so not outstanding). I think adding this detail about where the expression goes will help.