Update detab.c (Fixed #43)

[berkenvr]

The incorrect solution for exercise 1.20 has been corrected.
For detailed discussion, please refer to the issue #43.

[ohkimur]

Thanks for the follow-up and for proposing a change. After our discussion in issue #43, I think this PR still adds complexity without fixing the core problem.

• The exercise requires expanding actual tab characters ('\t') to the next tab stop, not interpreting the two-character sequence “\” + “t”.
• Replacing every tab with a fixed 8 spaces is still incorrect mid-line; the number of spaces must depend on the current column.

Suggestion to align with K&R 1.20:

• Track the current column.
• On '\t', output TABSIZE - (col % TABSIZE) spaces and increment col by that amount.
• On '\n', reset col to 0.
• Everything else: emit the character and increment col.

This keeps the solution minimal, correct, and easy to reason about. If you update the PR with this approach (TABSIZE as a macro is fine), I’m happy to re-review.