docs: Update reactivity-computed-watchers.md

[Yukon123]

Description of Problem

Computed is a funtion rather than method.So does the Watch and WatchEffect

Proposed Solution

Change method into funtion

Additional Information

We can learn from :https://v3.vuejs.org/guide/composition-api-introduction.html#standalone-computed-properties

Similar to ref and watch, computed properties can also be created outside of a Vue component with the computed function imported from Vue.

Just like how we set up a watcher on the user property inside our component using the watch option, we can do the same using the watch function imported from Vue. It accepts 3 arguments:

And it does show on source code

export function computed<T>( getter: ComputedGetter<T>, debugOptions?: DebuggerOptions ): ComputedRef<T>

[skirtles-code]

Thanks. I concur, they are functions and not methods.