doc: fix fs.watch overflow option accepted value

[arhxam]

Description

The overflow option for fs.watch() / fsPromises.watch() is
documented as accepting 'ignore' or 'throw', but the implementation
validates the value against ['ignore', 'error'] in
lib/internal/fs/watchers.js:

validateOneOf(overflow, 'options.overflow', ['ignore', 'error']);

So a user who follows the docs and passes overflow: 'throw' gets an
error instead of the described behavior:

const { watch } = require('node:fs/promises');
const it = watch('.', { overflow: 'throw' });
await it.next();
// -> TypeError [ERR_INVALID_ARG_VALUE]: The property 'options.overflow'
//    must be one of: 'ignore', 'error'. Received 'throw'

The value that is actually accepted, 'error', throws
ERR_FS_WATCH_QUEUE_OVERFLOW on overflow, which matches the documented
"throw an exception" behavior. This updates the documentation to use
'error'. Documentation-only change; 'throw' and 'error' are the
same length so no reflowing is needed.