[Caffe2] [Easy] Fix comment on caffe2_serialize_using_bytes_as_holder to reflect correct types
Summary:
the logic is:
```
template <typename T>
typename std::enable_if<
std::is_same<T, bool>::value || std::is_same<T, uint8_t>::value ||
std::is_same<T, int8_t>::value || std::is_same<T, uint16_t>::value ||
std::is_same<T, int16_t>::value,
void>::type
```
Test Plan: N/A
Reviewed By: simpkins
Differential Revision: D28587311
fbshipit-source-id: 970c673a9c1256600ec8bdd5f9ca53333a60d588
