message: Fix README typo
  Resolves
diff: @@ -59,7 +59,7 @@ The code is known to work on the following compilers:
  - Clang/LLVM 6 (or later) on Windows (older versions may work - we haven't tested.)
  - Visual Studio 2019 Preview 4 (or later) on Windows, with some caveats due to range-v3's strict conformance requirements:
  - range-v3 needs `/std:c++17 /permissive-`
  - - range-v3 needs a fully conforming preprocessor, so `/experimental:preprocesor` is necessary. Note that the conforming preprocessor diagnoses `C5105` "macro expansion producing 'defined' has undefined behavior" in some of the Windows SDK headers, so you'll probably want to suppress that warning with `/wd5105`.
  + - range-v3 needs a fully conforming preprocessor, so `/experimental:preprocessor` is necessary. Note that the conforming preprocessor diagnoses `C5105` "macro expansion producing 'defined' has undefined behavior" in some of the Windows SDK headers, so you'll probably want to suppress that warning with `/wd5105`.
  [ Note: We've "retired" support for Clang/C2 with the VS2015 toolset (i.e., the `v140_clang_c2` toolset) which Microsoft no longer supports for C++ use. We no longer have CI runs, but haven't gone out of our way to break anything, so it will likely continue to work. ]
  