docs(README): fix options table formatting (#478)

Author: WangZhao

README.md

@@ -68,7 +68,7 @@ new ExtractTextPlugin(options: filename | object)
 |Name|Type|Description|
 |:--:|:--:|:----------|
 |**`id`**|`{String}`|Unique ident for this plugin instance. (For advanced usage only, by default automatically generated)|
-|**`filename`**|`{String|Function}`|Name of the result file. May contain `[name]`, `[id]` and `[contenthash]`|
+|**`filename`**|`{String\|Function}`|Name of the result file. May contain `[name]`, `[id]` and `[contenthash]`|
 |**`allChunks`**|`{Boolean}`|Extract from all additional chunks too (by default it extracts only from the initial chunk(s))|
 |**`disable`**|`{Boolean}`|Disables the plugin|
 |**`ignoreOrder`**|`{Boolean}`|Disables order check (useful for CSS Modules!), `false` by default|