[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 <gurwls223@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

README.md

@@ -25,7 +25,9 @@ This README file only contains basic setup instructions.
 Spark is built using [Apache Maven](https://maven.apache.org/).
 To build Spark and its example programs, run:
 
-    ./build/mvn -DskipTests clean package
+```bash
+./build/mvn -DskipTests clean package
+```
 
 (You do not need to do this if you downloaded a pre-built package.)
 
@@ -38,28 +40,38 @@ For general development tips, including info on developing Spark using an IDE, s
 
 The easiest way to start using Spark is through the Scala shell:
 
-    ./bin/spark-shell
+```bash
+./bin/spark-shell
+```
 
 Try the following command, which should return 1,000,000,000:
 
-    scala> spark.range(1000 * 1000 * 1000).count()
+```scala
+scala> spark.range(1000 * 1000 * 1000).count()
+```
 
 ## Interactive Python Shell
 
 Alternatively, if you prefer Python, you can use the Python shell:
 
-    ./bin/pyspark
+```bash
+./bin/pyspark
+```
 
 And run the following command, which should also return 1,000,000,000:
 
-    >>> spark.range(1000 * 1000 * 1000).count()
+```python
+>>> spark.range(1000 * 1000 * 1000).count()
+```
 
 ## Example Programs
 
 Spark also comes with several sample programs in the `examples` directory.
 To run one of them, use `./bin/run-example <class> [params]`. For example:
 
-    ./bin/run-example SparkPi
+```bash
+./bin/run-example SparkPi
+```
 
 will run the Pi example locally.
 
@@ -70,7 +82,9 @@ locally with one thread, or "local[N]" to run locally with N threads. You
 can also use an abbreviated class name if the class is in the `examples`
 package. For instance:
 
-    MASTER=spark://host:7077 ./bin/run-example SparkPi
+```bash
+MASTER=spark://host:7077 ./bin/run-example SparkPi
+```
 
 Many of the example programs print usage help if no params are given.
 
@@ -79,7 +93,9 @@ Many of the example programs print usage help if no params are given.
 Testing first requires [building Spark](#building-spark). Once Spark is built, tests
 can be run using:
 
-    ./dev/run-tests
+```bash
+./dev/run-tests
+```
 
 Please see the guidance on how to
 [run tests for a module, or individual tests](https://spark.apache.org/developer-tools.html#individual-tests).