docs: fixes in 3.5 updates on reactivity-api page

[AloisSeckar]

Description of Problem

1. In 66a8ced example for "side effect cleanup" got duplicated for watchEffect (lines 281 and 318)
2. The example for "Pausing / resuming the watcher" shows watchEffect instead of watch

Proposed Solution

1. Remove the duplicated text
2. Fix the function name

Additional Information

1. I removed the original occurence to maintain same order of examples as for watch function listed later on the page

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