Fix typos in several old articles

'Relativistic Baseball' -- fix #488.
'Droppings' -- fix #252.
'Raindrop' -- fix #382, fix #378.
'Laser Pointer' -- fix #328.
'Short Answers' -- my own fixes.
'FedEx Bandwidth' -- my own fixes.
'Hockey Puck' -- fix #23, fix 307.