Fixes #6824 - Correct scan_number() grammar

Show `int_suffix_size` to appear 0 or 1 times
Show `exponent | . dec_lit` to appear 0 or 1 times, to show why `3f;` is a valid number
auto merge of #6825 : caitp/rust/issue-6824, r=Aatch

Show `int_suffix_size` to appear 0 or 1 times
Show `exponent | . dec_lit` to appear 0 or 1 times, to show why `3f;` is a valid number