Update test-collection.md

[oxbau]

Hello,
I identified some grammatical errors in this section. These are:

1. In the sentence "However, adding any arguments to function signature will enable fuzz testing for this test case," it should be "to the function signature" instead of "to function signature".
   
2. In the sentence "For example, for a package structured this way," it should be "For example, for a package structured in this way".

Thanks.

Closes #

Introduced changes

Checklist

• Linked relevant issue
• Updated relevant documentation
• Added relevant tests
• Performed self-review of the code
• Added changes to CHANGELOG.md

[cptartur]

We are no longer accepting contributions related to just spelling & grammar, see https://github.com/foundry-rs/starknet-foundry/blob/master/CONTRIBUTING.md#contributions-related-to-spelling-and-grammar