doc: added cpp highlight for string.md

There is no cpp highlight for some function in string.md file.

PR-URL: #329
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Nicola Del Gobbo <nicoladelgobbo@NickNaso.local>

doc/string.md

@@ -12,7 +12,7 @@ If an error occurs, a `Napi::Error` will get thrown. If C++ exceptions are not
 being used, callers should check the result of `Env::IsExceptionPending` before
 attempting to use the returned value.
 
-```
+```cpp
 String(napi_env env, napi_value value); ///< Wraps a N-API value primitive.
 ```
 - `[in] env` - The environment in which to create the string.
@@ -83,4 +83,4 @@ Returns a UTF-8 encoded C++ string.
 std::u16string Utf16Value() const;
 ```
 
-Returns a UTF-16 encoded C++ string.
+Returns a UTF-16 encoded C++ string.