Update function names declared upfront in pegthing

[serixscorpio]

While I was going through the pegthing example (web and ebook version), I noticed the function names in

(declare successful-move prompt-move game-over query-rows)

seem to be mismatched from the remaining code example. After checking against https://github.com/flyingmachine/pegthing/blob/master/src/pegthing/core.clj, I changed to

(declare user-entered-valid-move user-entered-invalid-move prompt-move game-over prompt-rows)

in order to refer to those function names before they are defined. This PR reflects the corresponding change in the HTML. Thanks.