Fix docs/Coroutines.rst syntax highlighting on Linux

Summary:
s/code-block:: C++/code-block:: c++ in docs/Coroutines.rst .

Patch by Gor Nishanov!  Edited by Sanjoy to fix a missing s/C/c/.

Reviewers: sanjoy, rengolin

Differential Revision: https://reviews.llvm.org/D22832

llvm-svn: 276806