Fix typo in reading-and-writing-to-file.md

[h-marumoto]

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

I found a typo in the following document and fix it.
https://phpspreadsheet.readthedocs.io/en/latest/topics/reading-and-writing-to-file/#generating-excel-files-from-html-content

This PR fixes documentation typos only. No code changes included.

[oleibman]

Thank you for your contribution.