message: use C++17 consistently for DPC++ in README
diff: @@ -63,7 +63,7 @@ The Ginkgo DPC++ module has the following __additional__ requirements:
  * _OneAPI 2021.3+_
  * Set `dpcpp` as the `CMAKE_CXX_COMPILER`
  -* `c++17` is used to compile this module, while the rest of Ginkgo is compiled using `c++14`.
  +* `c++17` is used to compile Ginkgo
  * The following oneAPI packages should be available:
  * oneMKL
  * oneDPL
  