Fix docs/Coroutines.rst syntax highlighting on Linux

[GorNishanov]

Summary

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

Hi Sanjoy, can you submit the patch on my behalf? I don't have commits rights yet.

llvm/trunk/docs/Coroutines.rst

 =====================
 
 Coroutine Representation
 ------------------------
 
 Let's look at an example of an LLVM coroutine with the behavior sketched
 by the following pseudo-code.
 
-.. code-block:: C++
+.. code-block:: c++
 
   void *f(int n) {
      for(;;) {
        print(n++);
        <suspend> // returns a coroutine handle on first suspend
      }     
   } 
 
entry:
     ret i32 0
   }
 
 Multiple Suspend Points
 -----------------------
 
 Let's consider the coroutine that has more than one suspend point:
 
-.. code-block:: C++
+.. code-block:: c++
 
   void *f(int n) {
      for(;;) {
        print(n++);
        <suspend>
        print(-n);
        <suspend>
      }
.. code-block:: python
 
   def coroutine(n):
     for i in range(n):
       yield i
 
 Python frontend would inject two more suspend points, so that the actual code
 looks like this:
 
-.. code-block:: C
+.. code-block:: c
 
   void* coroutine(int n) {
     int current_value; 
     <designate current_value to be coroutine promise>
     <SUSPEND> // injected suspend point, so that the coroutine starts suspended
     for (int i = 0; i < n; ++i) {
       current_value = i; <SUSPEND>; // corresponds to "yield i"
     }
     <SUSPEND final=true> // injected final suspend point
   }
 
 and python iterator `__next__` would look like:
 
-.. code-block:: C++
+.. code-block:: c++
 
   int __next__(void* hdl) {
     coro.resume(hdl);
     if (coro.done(hdl)) throw StopIteration();
     return *(int*)coro.promise(hdl, 4, false);
   }
 
 Intrinsics

 The frontend can emit this intrinsic if its language rules allow for this 
 optimization.
 
 Example:
 """"""""
 Consider the following example. A coroutine takes two parameters `a` and `b`
 that has a destructor and a move constructor.
 
-.. code-block:: C++
+.. code-block:: c++
 
   struct A { ~A(); A(A&&); bool foo(); void bar(); };
 
   task<int> f(A a, A b) {
     if (a.foo())
       return 42;
 
     a.bar();