[MINOR][DOCS] Make code blocks pretty in README.md
[MINOR][DOCS] Make code blocks pretty in README.md

### What changes were proposed in this pull request?

This PR proposes to enable [syntax highlighting](https://docs.github.com/en/github/writing-on-github/working-with-advanced-formatting/creating-and-highlighting-code-blocks#syntax-highlighting) in code blocks of `README.md`.

### Why are the changes needed?

To make it easier to read.

### Does this PR introduce _any_ user-facing change?

No, dev-only.

**Before:**

<img width="865" alt="Screen Shot 2022-01-05 at 10 31 32 AM" src="https://user-images.githubusercontent.com/6477701/148146477-addc6d9f-4da8-4860-9ead-6baaec442e0b.png">

**After:**

<img width="1067" alt="Screen Shot 2022-01-05 at 10 31 21 AM" src="https://user-images.githubusercontent.com/6477701/148146464-dc3d942f-a857-493c-a438-ebf23aa4c069.png">

### How was this patch tested?

Manually tested via the GitHub viewer.

Closes #35103 from HyukjinKwon/minor-format.

Authored-by: Hyukjin Kwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>
Signed-off-by: Dongjoon Hyun <6ff48b33cbb828151035af58a92f18a39f1d1036@apache.org>

cloud-fan/spark,gengliangwang/spark,JoshRosen/spark,holdenk/spark,ueshin/apache-spark,WeichenXu123/spark,apache/spark,cloud-fan/spark,srowen/spark,mahak/spark,nchammas/spark,dongjoon-hyun/spark,xuanyuanking/spark,zzcclp/spark,hvanhovell/spark,rednaxelafx/apache-spark,ueshin/apache-spark,dongjoon-hyun/spark,nchammas/spark,dongjoon-hyun/spark,dongjoon-hyun/spark,zero323/spark,vinodkc/spark,rednaxelafx/apache-spark,wangyum/spark,xuanyuanking/spark,WeichenXu123/spark,rednaxelafx/apache-spark,mahak/spark,srowen/spark,apache/spark,nchammas/spark,gengliangwang/spark,mahak/spark,xuanyuanking/spark,witgo/spark,JoshRosen/spark,apache/spark,HyukjinKwon/spark,ueshin/apache-spark,holdenk/spark,zzcclp/spark,zero323/spark,shaneknapp/spark,gengliangwang/spark,zzcclp/spark,witgo/spark,cloud-fan/spark,zzcclp/spark,apache/spark,WeichenXu123/spark,xuanyuanking/spark,vinodkc/spark,witgo/spark,witgo/spark,shaneknapp/spark,srowen/spark,holdenk/spark,HyukjinKwon/spark,witgo/spark,holdenk/spark,HyukjinKwon/spark,wangyum/spark,zzcclp/spark,shaneknapp/spark,ueshin/apache-spark,wangyum/spark,zero323/spark,vinodkc/spark,witgo/spark,zero323/spark,hvanhovell/spark,rednaxelafx/apache-spark,dongjoon-hyun/spark,srowen/spark,xuanyuanking/spark,vinodkc/spark,xuanyuanking/spark,JoshRosen/spark,shaneknapp/spark,nchammas/spark,mahak/spark,dongjoon-hyun/spark,zero323/spark,zzcclp/spark,HyukjinKwon/spark,ueshin/apache-spark,zzcclp/spark,WeichenXu123/spark,zero323/spark,apache/spark,srowen/spark,WeichenXu123/spark,gengliangwang/spark,cloud-fan/spark,ueshin/apache-spark,vinodkc/spark,hvanhovell/spark,mahak/spark,HyukjinKwon/spark,xuanyuanking/spark,gengliangwang/spark,witgo/spark,holdenk/spark,zero323/spark,HyukjinKwon/spark,JoshRosen/spark,rednaxelafx/apache-spark,hvanhovell/spark,mahak/spark,ueshin/apache-spark,holdenk/spark,shaneknapp/spark,JoshRosen/spark,rednaxelafx/apache-spark,hvanhovell/spark,nchammas/spark,JoshRosen/spark,WeichenXu123/spark,shaneknapp/spark,nchammas/spark,mahak/spark,wangyum/spark,apache/spark,hvanhovell/spark,gengliangwang/spark,vinodkc/spark,wangyum/spark,JoshRosen/spark,hvanhovell/spark,cloud-fan/spark,cloud-fan/spark,nchammas/spark,holdenk/spark,vinodkc/spark,dongjoon-hyun/spark,wangyum/spark,srowen/spark,wangyum/spark,gengliangwang/spark,shaneknapp/spark,srowen/spark,apache/spark,WeichenXu123/spark,cloud-fan/spark,HyukjinKwon/spark,rednaxelafx/apache-spark