Fix comment. pp-trace can only handle source file at a time.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@193746 91177308-0d34-0410-b5e6-96231b3b80d8

pp-trace/PPTrace.cpp

@@ -18,7 +18,7 @@
 //
 // The pp-trace tool supports the following general command line format:
 //
-//    pp-trace [pp-trace options] (source file(s)) [compiler options]
+//    pp-trace [pp-trace options] (source file) [compiler options]
 //
 // Basically you put the pp-trace options first, then the source file or files,
 // and then any options you want to pass to the compiler.