message: update clang-format version in README
diff: @@ -55,7 +55,7 @@ The Ginkgo CUDA module has the following __additional__ requirements:
  In addition, if you want to contribute code to Ginkgo, you will also need the
  following:
  -* _clang-format 5.0.0+_ (ships as part of _clang_)
  +* _clang-format 8.0.0+_ (ships as part of _clang_)
  * _clang-tidy_ (optional, when setting the flag `-DGINKGO_WITH_CLANG_TIDY=ON`)
  * _iwyu_ (Include What You Use, optional, when setting the flag `-DGINKGO_WITH_IWYU=ON`)
  