[SPARK-28752][BUILD][DOCS][FOLLOW-UP] Render examples imported from Jekyll properly via Rouge

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

This PR proposes to use Pygment compatible format by Rouge. As of https://github.com/apache/spark/pull/26521, we use Rouge instead of Pygment wrapper in Ruby.
Rouge claims Pygment compatibility; and we should output as Pygment does.

```ruby
Rouge::Formatters::HTMLPygments.new(formatter)
```

wraps codes with `<div class="highlight"><pre>...` properly.

### Why are the changes needed?

To keep the documentation pretty and not broken.

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

Theoretically, no.

This is rather a regression fix in documentation (that happens only by https://github.com/apache/spark/pull/26521 in master). See the malformed doc in preview - https://spark.apache.org/docs/3.0.0-preview2/sql-pyspark-pandas-with-arrow.html

### How was this patch tested?

Manually built the doc.

**Before:**
![Screen Shot 2020-01-13 at 10 21 28 AM](https://user-images.githubusercontent.com/6477701/72229159-ba766a80-35ef-11ea-9a5d-9583448e7c1c.png)

**After:**

![Screen Shot 2020-01-13 at 10 26 33 AM](https://user-images.githubusercontent.com/6477701/72229157-b34f5c80-35ef-11ea-8b3a-492e8aa0f82a.png)

Closes #27182 from HyukjinKwon/SPARK-28752-followup.

Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

docs/_plugins/include_example.rb

@@ -54,9 +54,9 @@ module Jekyll
         puts(e.backtrace)
         exit 1
       end
-      code = select_lines(code)
+      code = select_lines(code).strip
 
-      formatter = Rouge::Formatters::HTML.new
+      formatter = Rouge::Formatters::HTMLPygments.new(Rouge::Formatters::HTML.new)
       lexer = Rouge::Lexer.find(@lang)
       rendered_code = formatter.format(lexer.lex(code))