update Math.sol path

[glesaint]

🚀 Description

MathMock contract imports a file that exists only if the "contracts" folder is named contracts.
This PR fixes it so it uses a relative path.

ISSUE

• If file is in: repo/contracts/openzeppelin/mocks/MathMock.sol
• It imports (fails) repo/contracts/contracts/math/Math.sol

FIX

• If file is in: repo/contracts/openzeppelin/mocks/MathMock.sol
• It imports (success) repo/contracts/openzeppelin/math/Math.sol

• 📘 I've reviewed the OpenZeppelin Contributor Guidelines
• [N/A?] ✅ I've added tests where applicable to test my new functionality.
• [N/A?] 📖 I've made sure that my contracts are well-documented.
• [N/A?] 🎨 I've run the JS/Solidity linters and fixed any issues (npm run lint:all:fix).

[nventuro]

Great @glesaint, thanks! I'll check if there are more instances of this.