[SPARK-48495][DOCS][FOLLOW-UP] Fix Table Markdown in Shredding.md

Minor change that shouldn't require a Jira to fix the unbalanced row in the example of Shredding.md

Closes #47407 from RussellSpitzer/patch-1.

Authored-by: Russell Spitzer <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

common/variant/shredding.md

@@ -39,7 +39,7 @@ optional group variant_col {
 ```
 
 | Variant Value | Top-level untyped_value | b.untyped_value | Non-null in a | Non-null in b.c |
-|---------------|--------------------------|---------------|---------------|
+|---------------|--------------------------|---------------|---------------|------------------|
 | {a: 123, b: {c: “hello”}} | null | null | typed_value | typed_value |
 | {a: 1.23, b: {c: “123”}} | null | null | untyped_value | typed_value |
 | {a: [1,2,3], b: {c: null}} | null | null | untyped_value | untyped_value |