Add "opengles" in the helper information of Dawn samples

BUG=dawn:707

Change-Id: Id45deb13d8afeef985d81ebc3f7a1a80040498bf
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/44660
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/examples/SampleUtils.cpp b/examples/SampleUtils.cpp
index 59ab852..f344825 100644
--- a/examples/SampleUtils.cpp
+++ b/examples/SampleUtils.cpp
@@ -250,7 +250,7 @@
         }
         if (std::string("-h") == argv[i] || std::string("--help") == argv[i]) {
             printf("Usage: %s [-b BACKEND] [-c COMMAND_BUFFER]\n", argv[0]);
-            printf("  BACKEND is one of: d3d12, metal, null, opengl, vulkan\n");
+            printf("  BACKEND is one of: d3d12, metal, null, opengl, opengles, vulkan\n");
             printf("  COMMAND_BUFFER is one of: none, terrible\n");
             return false;
         }