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': {}}
```
____

Author: ahmetmeleq

CHANGELOG.md

@@ -1,11 +1,12 @@
-## 0.18.6-dev0
+## 0.18.6
 
 ### Enhancements
 
 ### Features
 
 ### Fixes
 - **Improved epub partition errors** EPUB partition will now produce new type of error on unprocessable files.
+- **Fix type for serialized TableChunks** Use `TableChunk` for the string value of the field `type` when serializing elements of type `TableChunk`, rather than using the value `Table`.
 
 ## 0.18.5
 

unstructured/__version__.py

@@ -1 +1 @@
-__version__ = "0.18.6-dev0"  # pragma: no cover
+__version__ = "0.18.6"  # pragma: no cover

unstructured/documents/elements.py

@@ -943,7 +943,7 @@ class Table(Text):
 class TableChunk(Table):
     """An element for capturing chunks of tables."""
 
-    category = "Table"
+    category = "TableChunk"
 
 
 class Header(Text):