Use the correct pop pragma

BUG=tint:43

Change-Id: If0f6ff0b65af4c66fe7114eb7160e93ef060fbf9
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/19104
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: dan sinclair <dsinclair@google.com>

samples/main.cc

@@ -143,7 +143,7 @@ bool ParseArgs(const std::vector<std::string>& args, Options* opts) {
 #pragma clang diagnostic ignored "-Wunused-template"
 template <typename T>
 bool ReadFile(const std::string& input_file, std::vector<T>* buffer) {
-#pragma Clang pop
+#pragma clang diagnostic pop
 
   if (!buffer) {
     std::cerr << "The buffer pointer was null" << std::endl;