docs: fix typo in documentation for seed option (#285)

Author: kamalaknn

README.md

@@ -149,7 +149,7 @@ The default value for this option is a regular expression targeting Webpack's [d
 Type: `Object`<br>
 Default: `{}`
 
-A cache of key/value pairs to used to seed the manifest. This may include a set of [custom key/value](https://developer.mozilla.org/en-US/Add-ons/WebExtensions/manifest.json) pairs to include in your manifest, or may be used to combine manifests across compilations in [multi-compiler mode](https://github.com/webpack/webpack/tree/master/examples/multi-compiler). To combine manifests, pass a shared seed object to each compiler's `WebpackManifestPlugin` instance.
+A cache of key/value pairs used to seed the manifest. This may include a set of [custom key/value](https://developer.mozilla.org/en-US/Add-ons/WebExtensions/manifest.json) pairs to include in your manifest, or may be used to combine manifests across compilations in [multi-compiler mode](https://github.com/webpack/webpack/tree/master/examples/multi-compiler). To combine manifests, pass a shared seed object to each compiler's `WebpackManifestPlugin` instance.
 
 ### `serialize`