Fix some typos found

What does this implement/fix?

This MR fixes some typos in English text that I found using a spell checker.

@cantonios, @sciencewhiz: Could you please review my changes?

Reference issue

See also (discussion in) !660 (merged).

Additional information

There are also some occurrences of "assignement" which should be changed to "assignment", e.g. in lines 52, 56, 58, 87, 140, and 144 of unsupported/test/cxx11_tensor_builtins_sycl.cpp.