[SPARK-52080][DOCS][FOLLOWUP] Fix the heading level of Daily Build Pipeline Status

### What changes were proposed in this pull request?
This pr fix the heading level of `Daily Build Pipeline Status`.

### Why are the changes needed?
Fix the heading level

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Manual check

### Was this patch authored or co-authored using generative AI tooling?
No

Closes #50899 from LuciferYang/SPARK-52080-FOLLOWUP-2.

Authored-by: yangjie01 <[email protected]>
Signed-off-by: Ruifeng Zheng <[email protected]>

Author: LuciferYang

README.md

@@ -21,7 +21,7 @@ You can find the latest Spark documentation, including a programming
 guide, on the [project web page](https://spark.apache.org/documentation.html).
 This README file only contains basic setup instructions.
 
-# Daily Build Pipeline Status
+## Daily Build Pipeline Status
 
 | Branch     | Status                                                                                                                                                                                                          | Desc |
 |------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------|