[MINOR][SQL][TEST] Include usage example for generating output for si…

…ngle test in SQLQueryTestSuite

## What changes were proposed in this pull request?
This is a very minor pr to include the usage example to generate output for single test in SQLQueryTestSuite. I tried to deduce it from the existing example and ran into a scenario
where sbt is simply looping to run the same test over and over again. Here is the example
of running a single test.

```
build/sbt "~sql/test-only *SQLQueryTestSuite -- -z inline-table.sql"
```
I tried to generate the output for a single test by prepending `SPARK_GENERATE_GOLDEN_FILES=1` like following
```
SPARK_GENERATE_GOLDEN_FILES=1 build/sbt "~sql/test-only *SQLQueryTestSuite -- -z describe.sql"
```
In this case i found that sbt is looping trying to run describe.sql over and over again as we are running the test in on continuous mode (because of `~` prefix ) where it detects a change in
the generated result file which in turn triggers a build and test. I have included an example where
we don't run it in continuous mode when generating the output. Hopefully it saves other developers some time.
## How was this patch tested?
Verified manually in my dev setup.

Closes #23995 from dilipbiswal/dkb_sqlquerytest_usage.

Authored-by: Dilip Biswal <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

sql/core/src/test/scala/org/apache/spark/sql/SQLQueryTestSuite.scala

@@ -48,11 +48,16 @@ import org.apache.spark.sql.types.StructType
  *   build/sbt "~sql/test-only *SQLQueryTestSuite -- -z inline-table.sql"
  * }}}
  *
- * To re-generate golden files, run:
+ * To re-generate golden files for entire suite, run:
  * {{{
  *   SPARK_GENERATE_GOLDEN_FILES=1 build/sbt "sql/test-only *SQLQueryTestSuite"
  * }}}
  *
+ * To re-generate golden file for a single test, run:
+ * {{{
+ *   SPARK_GENERATE_GOLDEN_FILES=1 build/sbt "sql/test-only *SQLQueryTestSuite -- -z describe.sql"
+ * }}}
+ *
  * The format for input files is simple:
  *  1. A list of SQL queries separated by semicolon.
  *  2. Lines starting with -- are treated as comments and ignored.