docs: Correct code example for kv variable in manual

[mohsinalipro]

This commit addresses a minor issue in the Deno documentation manual related to the kv variable. The code example provided had an incorrect reference to the variable used to open the key-value store. The variable should be kv, not db.

Changes Made:

• Corrected the variable name from db to kv in the code example.
• Verified the correctness of the code snippet.

[kwhinnery]

thank you!