fix(export/markdown): avoid over-stripping indentation in ranged tags

[YsuOS]

Use math.min to clamp the strip amount to the actual leading whitespace, preventing striping non-whitespace characters. when a line has less indentation than tag_indent.

[YsuOS]

Test result

• Src norg file

$ cat /tmp/test.norg
* Title
  - bullet point
  @code
ABCDEFG
abcdefg
  @end
  @code
  ABCDEFG
  abcdefg
  @end
  @code
    ABCDEFG
    abcdefg
  @end

• Actual result: the "ab" is stripped in the first code block because this line start from column 0 but the tag_indent is 2.

$ cat /tmp/test-before.md
# Title

- bullet point
```
ABCDEFG
cdefg  <<<--- HERE!!
```
```
ABCDEFG
abcdefg
```
```
  ABCDEFG
  abcdefg
```

• Expected result (with this patch)

$ cat /tmp/test-after.md
# Title

- bullet point
```
ABCDEFG
abcdefg
```
```
ABCDEFG
abcdefg
```
```
  ABCDEFG
  abcdefg
```