Fix integer constant token

[gpetrousov]

Hey, thanks for the tutorial series.
I found a small bug in one of the tokens in the parser.

parser.y:124.11-16: error: symbol ‘ICONST’ is used, but is not defined as a token and has no rules; did you mean ‘CONST’?
  124 | constant: ICONST | FCONST | CCONST ;
      |           ^~~~~~
      |           CONST
cc1: fatal error: parser.tab.c: No such file or directory
compilation terminated.
lexer.l:8:18: fatal error: parser.tab.h: No such file or directory
    8 |         #include "parser.tab.h"
      |                  ^~~~~~~~~~~~~~
compilation terminated.