Update API doc comment

Author: jperals

src/__tests__/snapshot-tests/__snapshots__/documenter.test.ts.snap

@@ -21964,7 +21964,7 @@ exports[`Components definition for split-panel matches the snapshot: split-panel
       "type": "string",
     },
     {
-      "description": "Header of the split panel.",
+      "description": "Header text of the split panel.",
       "name": "header",
       "optional": false,
       "type": "string",

src/split-panel/interfaces.ts

@@ -6,7 +6,7 @@ import { BaseComponentProps } from '../internal/base-component';
 
 export interface SplitPanelProps extends BaseComponentProps {
   /**
-   * Header of the split panel.
+   * Header text of the split panel.
    */
   header: string;
   children: React.ReactNode;