doc: fix return type of ArrayBuffer::Data

PR-URL: #552
Reviewed-By: NickNaso <nicoladelgobbo@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>

doc/array_buffer.md

@@ -123,7 +123,7 @@ Returns the length of the wrapped data, in bytes.
 ### Data
 
 ```cpp
-T* Napi::ArrayBuffer::Data() const;
+void* Napi::ArrayBuffer::Data() const;
 ```
 
 Returns a pointer the wrapped data.