Fix Incorrect Comment in String Indexing Example

[fancling]

Hello,

I've noticed a small error in the comments of the string indexing example provided in the [src/data_types/test_strings.py]. The comment incorrectly refers to the sixth character of the string as the "Fifth character".

Current Code:
word = 'Python'
assert word[0] == 'P' # First character.
assert word[5] == 'n' # Fifth character.
Issue:
The comment for assert word[5] == 'n' incorrectly states "Fifth character" while it actually checks the sixth character of the string word.

Suggested Correction:
word = 'Python'
assert word[0] == 'P' # First character.
assert word[5] == 'n' # Sixth character.
Correction Explanation:
Python strings are zero-indexed, so the first character is at index 0, and the sixth character is at index 5. The proposed change corrects the comment to accurately reflect the character's position in the string.

I hope this helps improve the documentation/code for future readers. If it's okay, I can submit a pull request #83 with this minor fix.

Thank you for your attention to this matter.

Best regards,
fancling