Match doc example to Blazor template code (#18149)

Author: guardrex

aspnetcore/blazor/get-started/samples_snapshot/3.x/Counter1.razor

@@ -2,15 +2,15 @@
 
 <h1>Counter</h1>
 
-<p>Current count: @_currentCount</p>
+<p>Current count: @currentCount</p>
 
 <button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
 
 @code {
-    private int _currentCount = 0;
+    private int currentCount = 0;
 
     private void IncrementCount()
     {
-        _currentCount++;
+        currentCount++;
     }
 }

aspnetcore/blazor/get-started/samples_snapshot/3.x/Counter2.razor

@@ -2,18 +2,18 @@
 
 <h1>Counter</h1>
 
-<p>Current count: @_currentCount</p>
+<p>Current count: @currentCount</p>
 
 <button class="btn btn-primary" @onclick="IncrementCount">Click me</button>
 
 @code {
-    private int _currentCount = 0;
+    private int currentCount = 0;
 
     [Parameter]
     public int IncrementAmount { get; set; } = 1;
 
     private void IncrementCount()
     {
-        _currentCount += IncrementAmount;
+        currentCount += IncrementAmount;
     }
 }