docs: fix minor typos and grammar in user operations guide

[mdqst]

spotted a few issues:

• added the missing “to” in how you can use it to work with user operations.
• removed the extra slash in the URL: https://www.alchemy.com/blog/account-abstraction.
• corrected grammar: changed which makes it possible → which make it possible to match the plural subject.

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• Did you add new tests and confirm existing tests pass? (yarn test)
• Did you update relevant docs? (docs are found in the site folder, and guidelines for updating/adding docs can be found in the contribution guide)
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