Fix SampleApp instructions in the Windows quick-start

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1661553002
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=1661553002

Review URL: https://codereview.chromium.org/1661553002

site/user/quick/windows.md

@@ -130,8 +130,8 @@ Build and run SampleApp in Visual Studio
 6.  Once the build is complete, you should see a window with various
     example graphics. To move through the sample app, use the
     following keypresses:
-    -    right-arrow key: cycle through different test pages
-    -    left-arrow key: cycle through rendering methods for each test page
+    -    right- and left-arrow key: cycle through different test pages
+    -    'D' key: cycle through rendering methods for each test page
     -    other keys are defined in SampleApp.cpp’s
          SampleWindow::onHandleKey() and SampleWindow::onHandleChar()
          methods