style: improve heading consistency with singular form

[Earnwithalee7890]

What changed? Why?

This PR improves heading consistency by changing "Deploy Your Contracts" to "Deploy Your Contract" to accurately reflect the content of the guide, which demonstrates deploying a single contract.

Specific changes:

• docs/base-chain/quickstart/deploy-on-base.mdx - Changed heading from plural "Contracts" to singular "Contract"

Why this change:

• Accuracy - The guide walks through deploying one contract (Counter.sol), not multiple contracts
• Clarity - Singular form sets proper expectations for what the section covers
• Consistency - Heading now matches the actual content and examples
• User experience - Accurate headings help users quickly understand what they'll learn

Throughout the guide, the text refers to deploying "your contract" (singular), making the plural heading inconsistent with the content.

Notes to reviewers

• This is a minor wording change for accuracy
• No changes to instructions, code examples, or functionality
• The section content remains unchanged; only the heading was updated
• Improves the accuracy of the table of contents
• No impact on documentation functionality

How has it been tested?

• Manually verified the heading accurately reflects the section content
• Confirmed documentation builds successfully without errors
• Validated that the table of contents displays correctly
• Checked that the updated heading makes sense in context
• Verified that all internal references and navigation still work correctly

[cb-heimdall]

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