docs: fix typos in typings, tests, and core comments (addons only)

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

addons/addon-clipboard/typings/addon-clipboard.d.ts

@@ -82,7 +82,7 @@ declare module '@xterm/addon-clipboard' {
   /**
    * The clipboard provider interface that enables xterm.js to access the browser clipboard.
    *
-   * Note this this clipboard provider does not implement own cut buffers as xterm does,
+   * Note that this clipboard provider does not implement own cut buffers as xterm does,
    * but redirect the selection parameter always to navigator.clipboard.
    */
   export class BrowserClipboardProvider implements IClipboardProvider{

addons/addon-webgl/typings/addon-webgl.d.ts

@@ -23,12 +23,12 @@ declare module '@xterm/addon-webgl' {
     public readonly onChangeTextureAtlas: IEvent<HTMLCanvasElement>;
 
     /**
-     * An event that is fired when the a new page is added to the texture atlas.
+     * An event that is fired when a new page is added to the texture atlas.
      */
     public readonly onAddTextureAtlasCanvas: IEvent<HTMLCanvasElement>;
 
     /**
-     * An event that is fired when the a page is removed from the texture atlas.
+     * An event that is fired when a page is removed from the texture atlas.
      */
     public readonly onRemoveTextureAtlasCanvas: IEvent<HTMLCanvasElement>;