The link above has instructions too.
The (C) repair cap with the red rubber seal sells for $2.47 at Lowes and can be changed out in less than a minute without tools.
We normally have people turn the water off first,
Remove the cap
cup your hand over the fill valve and turn the water on slightly letting water flow into the tank.
This flushes the fill valve, there may be sand or grit that is preventing a good seal.
Turn water off, replace with new cap.