[Workers AI] Fix Speech-to-text pricing table

[eartharoid]

Summary

• Fixed the Speech-to-text pricing table
• Improved grammar on the line above

Screenshots (optional)

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[KianNH]

Thanks for fixing these!

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