Update performance.md

[DanielOX]

Grammatical fix. removed redundant will.

Type of PR

• BUG
• FEAT
• MAINT
• DOC

Is your Pull Request linked to an existing Issue or Pull Request?

No

Give a brief description for the solution you have provided

• Fixed very minor grammatical error. Removed redundant will word in topic_guides/blocking/model_training.html
• Fixed spelling mistake from beteween to between in topic_guides/comparisons/customising_comparisons.html

PR Checklist

• Added documentation for changes
• Added feature to example notebooks or tutorial (if appropriate)
• Added tests (if appropriate)
• Updated CHANGELOG.md (if appropriate)
• Made changes based off the latest version of Splink
• [] Run the linter

[RobinL]

Thanks!