fix: type for serialized TableChunks (#4056)
#### To test, simply serialize a TableChunk element with and without the
changes in the PR
____
**Without the changes:**
```
In [1]: from unstructured.documents.elements import TableChunk
In [2]: TableChunk("hi")
Out[2]: <unstructured.documents.elements.TableChunk at 0x110113410>
In [3]: TableChunk("hi").to_dict()
Out[3]:
{'type': 'Table',
'element_id': '6267e99a-46d8-4f2d-a206-51c691469c72',
'text': 'hi',
'metadata': {}}
```
____
**With the changes:**
```
In [1]: from unstructured.documents.elements import TableChunk
In [2]: TableChunk("hi")
Out[2]: <unstructured.documents.elements.TableChunk at 0x10367f050>
In [3]: TableChunk("hi").to_dict()
Out[3]:
{'type': 'TableChunk',
'element_id': 'f91af3ac-0dea-4dc4-8a6a-69c28cfcca3b',
'text': 'hi',
'metadata': {}}
```
____
