feat: enable sticky file headers by default

Update the stickyFileHeaders default value to `true` so that it is
enabled by default. Also correct the name of this option in the
documentation by adding the missing trailing "s".

README.md

@@ -156,7 +156,7 @@ draw(): void
 synchronisedScroll(): void
 fileListToggle(startVisible: boolean): void
 highlightCode(): void
-stickyFileHeader(): void
+stickyFileHeaders(): void
 ```
 
 ### Diff2HtmlUI Configuration
@@ -166,7 +166,7 @@ stickyFileHeader(): void
 - `fileListToggle`: allow the file summary list to be toggled: `true` or `false`, default is `true`
 - `fileListStartVisible`: choose if the file summary list starts visible: `true` or `false`, default is `false`
 - `fileContentToggle`: allow each file contents to be toggled: `true` or `false`, default is `true`
-- `stickyFileHeader`: make file headers sticky: `true` or `false`, default is `false`
+- `stickyFileHeaders`: make file headers sticky: `true` or `false`, default is `true`
 - [All the options](#diff2html-configuration) from Diff2Html are also valid configurations in Diff2HtmlUI
 
 ### Diff2HtmlUI Browser

src/ui/js/diff2html-ui-base.ts

@@ -32,7 +32,7 @@ export const defaultDiff2HtmlUIConfig = {
    */
   smartSelection: true,
   fileContentToggle: true,
-  stickyFileHeaders: false,
+  stickyFileHeaders: true,
 };
 
 export class Diff2HtmlUI {