Use Markdown syntax coloring for code snippets (#259)

index.bs

@@ -311,7 +311,7 @@ An <dfn export>OperatingPointSelectorProperty</dfn> may be associated with an [=
 
 <h6 id="operating-point-selector-property-syntax" class="no-toc">Syntax</h6>
 
-```
+```c
 class OperatingPointSelectorProperty extends ItemProperty('a1op') {
     unsigned int(8) op_index;
 }
@@ -345,7 +345,7 @@ The <code>[=AV1LayeredImageIndexingProperty=]</code> documents the size in bytes
 
 <h6 id="layered-image-indexing-property-syntax" class="no-toc">Syntax</h6>
 
-```
+```c
 class AV1LayeredImageIndexingProperty extends ItemProperty('a1lx') {
     unsigned int(7) reserved = 0;
     unsigned int(1) large_size;