docs: fix a/an grammar errors in documentation

[serikssonn]

Description:
This PR fixes minor grammar errors found in the documentation:

Documentation fixes:

• Fixed "an the" → "the" in storage-vec documentation
• Fixed "an ContractId" → "a ContractId" (ContractId starts with consonant sound)
• Fixed "an Contract" → "a Contract"

---

Note

Fixes minor a/an grammar errors across mint-to-contract, ContractId, and StorageVec documentation pages.

• Documentation:
  • Grammar fixes (a/an usage):
    • operations/asset/mint/contract.md: "an Contract" → "a Contract".
    • operations/namespace/contract-id.md: "an ContractId" → "a ContractId".
    • operations/storage/libraries/storage-vec.md: "an the" → "the".

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[fuel-cla-bot]

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