[editor] Add special character 'apostrophe'

projects/editor/src/app/text-editor/rich-text-editor.component.html

@@ -195,6 +195,7 @@
     <mat-menu #specialCharsMenu="matMenu" yPosition="above">
       <button mat-button (click)="insertSpecialChar('&nbsp;')"
               [matTooltip]="'Geschütztes Leerzeichen'">&blank;</button>
+      <button mat-button (click)="insertSpecialChar('&rsquo;')">&rsquo;</button>
       <button mat-button (click)="insertSpecialChar('&bdquo;')">&bdquo;</button>
       <button mat-button (click)="insertSpecialChar('&rdquo;')">&rdquo;</button>
       <button mat-button (click)="insertSpecialChar('&female;')">&female;</button>