doc: fix typo in readme

[LinusU]

Without this:

(!) You have passed an unrecognized option
Unknown input option: externals. Allowed options: acorn, acornInjectPlugins, cache, chunkGroupingSize, context, experimentalCacheExpiry, experimentalOptimizeChunks, experimentalTopLevelAwait, external, inlineDynamicImports, input, manualChunks, moduleContext, onwarn, perf, plugins, preserveModules, preserveSymlinks, shimMissingExports, strictDeprecations, treeshake, watch

[lukastaegert]

Thanks for spotting!