Fix typos and improve grammar in documentation files

[Dimitrolito]

This pull request addresses several minor typos and grammatical improvements across multiple documentation files.

Changes made:

• Corrected the article "A" to "An" in the pindexer README file, where it was incorrectly used before a word starting with a vowel sound.
• Fixed a spelling issue in the spend_key.md file, where "organizational" was written without the hyphen.
• Updated the routing.md file to fix a similar article issue, changing "a index" to "an index."
• Corrected missing hyphen in the compound adjective "right hand" in contributions.md.

Issue Ticket:

• No related ticket.

Testing:

• Review the changes in the respective markdown files and ensure that the grammar and spelling adjustments align with standard English conventions.

Checklist before requesting a review:

• I have added guiding text to explain how a reviewer should test these changes.
• If this code contains consensus-breaking changes, I have added the "consensus-breaking" label. Otherwise, I declare my belief that there are not consensus-breaking changes.

[erwanor]

Thanks!