fix typos in Update contracts.md

[moshderte]

This PR improves the quality and clarity of the core_contracts.md documentation by:

imumutable ---> immutable

responsibilites ---> responsibilities

transfering ---> transferring

inputing ---> inputting

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[aquint-zama]

@cla-bot check

[cla-bot[bot]]

The cla-bot has been summoned, and re-checked this pull request!

[moshderte]

@aquint-zama approve pls