Remove explicit highlighting as pygments doesn't seem to be installed

pep-0556.rst

@@ -128,7 +128,7 @@ or threaded.
 
 An internal structure ``gc_mutex`` is added to avoid two GC runs at once:
 
-.. code-block:: c
+.. code-block::
 
    static struct {
        PyThread_type_lock collecting;  /* taken when collecting */
@@ -139,7 +139,7 @@ An internal structure ``gc_mutex`` is added to avoid two GC runs at once:
 An internal structure ``gc_thread`` is added to handle synchronization with
 the GC thread:
 
-.. code-block:: c
+.. code-block::
 
    static struct {
       PyThread_type_lock wakeup; /* acts as an event
@@ -174,7 +174,7 @@ Here is a proposed pseudo-code for the main primitives, public and internal,
 required for implementing this PEP.  All of them will be implemented in C
 and live inside the ``gc`` module, unless otherwise noted:
 
-.. code-block:: python
+.. code-block::
 
    def collect_with_callback(generation):
        """