Update CONTRIBUTING.md to using c++ highlight where applicable.

CONTRIBUTING.md

@@ -76,13 +76,13 @@ Template implementations should end in `.i.hh`.
 
 ## Class naming
 Classes should be **CamelCase**.
-```
+```c++
 class CommandLine
 {
 };
 ```
 Class members should be prefixed with **m** and be **lazyCamelCase**.
-```
+```c++
 class CommandLine
 {
 private:
@@ -92,7 +92,7 @@ private:
 ```
 Class methods should be **lazyCamelCase**.
 Class member getter methods should be the members name **lazyCamelCase**  `int argc() const`.
-```
+```c++
 class Commandline
 {
 private: