Fix std.fmt.Parser.peek compiler error from #20505

[gitabaz]

Simple fix to the compiler error in std.fmt.Parser.peek:

error: variable of type 'comptime_int' must be const or comptime
        var i = 0;
            ^
note: to modify this variable at runtime, it must be given an explicit fixed-size number type

[Rexicon226]

duplicate PR of #20532