[clangd] Fix a typo in TUSchedulerTests

[ilya-biryukov]

LGTM

unittests/clangd/TUSchedulerTests.cpp

auto DoUpdate = [&](std::string Contents) -> bool {
     return Updated;
   };
 
   // Test that subsequent updates with the same inputs do not cause rebuilds.
   ASSERT_TRUE(DoUpdate(SourceContents));
   ASSERT_FALSE(DoUpdate(SourceContents));
 
   // Update to a header should cause a rebuild, though.
-  Files[Header] = time_t(1);
+  Timestamps[Header] = time_t(1);
   ASSERT_TRUE(DoUpdate(SourceContents));
   ASSERT_FALSE(DoUpdate(SourceContents));
 
   // Update to the contents should cause a rebuild.
   auto OtherSourceContents = R"cpp(
       #include "foo.h"
       int c = d;
     )cpp";