HParams: Reduce Header Cell padding (#6469)

## Motivation for features / changes
In #6449 we moved the padding between cells to be on each header instead
of in the table component. However, we mistakenly doubled the padding by
adding 4px to each cell instead of 2 to each. This PR remedies that
mistake.

## Screenshots of UI changes (or N/A)
Before:
<img width="354" alt="Screenshot 2023-06-27 at 10 24 43 PM"
src="https://github.com/tensorflow/tensorboard/assets/8672809/fed0ac53-a4c8-43fb-98c4-66301796e1da">

After:
<img width="363" alt="Screenshot 2023-06-27 at 10 24 12 PM"
src="https://github.com/tensorflow/tensorboard/assets/8672809/772bae17-d30a-4a4f-b482-508d4021cfd9">

Author: JamesHollyer

tensorboard/webapp/widgets/data_table/header_cell_component.scss

@@ -21,7 +21,7 @@ $_icon_size: 12px;
 
 :host {
   display: table-cell;
-  padding: 4px;
+  padding: 2px;
   vertical-align: bottom;
 
   &:hover {