Added alt text for github image

ExpenseTracker.Client/Shared/NavMenu.razor

@@ -56,7 +56,7 @@
                 </div>
                 <div class="github-logo">
                     <a href="https://github.com/syncfusion/blazor-showcase-expense-tracker" target="_blank">
-                        <img src="mark.svg" alt="Github source code" /><span>Source Code</span>
+                        <img src="mark.svg" alt="GitHub source code" /><span>Source Code</span>
                     </a>
                 </div>
             </div>