docs: correct variable name in build.lib.fileName option description (#18197)

Author: xxhls

docs/config/build-options.md

@@ -166,7 +166,7 @@ Options to pass on to [@rollup/plugin-dynamic-import-vars](https://github.com/ro
 - **Type:** `{ entry: string | string[] | { [entryAlias: string]: string }, name?: string, formats?: ('es' | 'cjs' | 'umd' | 'iife')[], fileName?: string | ((format: ModuleFormat, entryName: string) => string) }`
 - **Related:** [Library Mode](/guide/build#library-mode)
 
-Build as a library. `entry` is required since the library cannot use HTML as entry. `name` is the exposed global variable and is required when `formats` includes `'umd'` or `'iife'`. Default `formats` are `['es', 'umd']`, or `['es', 'cjs']`, if multiple entries are used. `fileName` is the name of the package file output, default `fileName` is the name option of package.json, it can also be defined as function taking the `format` and `entryAlias` as arguments.
+Build as a library. `entry` is required since the library cannot use HTML as entry. `name` is the exposed global variable and is required when `formats` includes `'umd'` or `'iife'`. Default `formats` are `['es', 'umd']`, or `['es', 'cjs']`, if multiple entries are used. `fileName` is the name of the package file output, default `fileName` is the name option of package.json, it can also be defined as function taking the `format` and `entryName` as arguments.
 
 ## build.manifest