Fix grammar: 'less then' to 'less than'

[Zentex1337]

Summary

• Fixed grammar error: 'less then' to 'less than' in test descriptions across multiple files:
  • waffle-mock-contract/README.md
  • waffle-mock-contract/test/amirichalready.test.ts
  • examples/mock-contracts/test/AmIRichAlready.test.ts
  • examples/called-on-contract/test/AmIRichAlready.test.ts
  • docs/source/mock-contract.rst

'then' is used for time sequence, 'than' is used for comparisons.

[changeset-bot]

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Latest commit: e05f501

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