[MINOR][SQL] Add prettyName for current_database function

## What changes were proposed in this pull request?
Added a `prettyname` for current_database function.

## How was this patch tested?
Manually.

Before:
```
scala> sql("select current_database()").show
+-----------------+
|currentdatabase()|
+-----------------+
|          default|
+-----------------+
```

After:
```
scala> sql("select current_database()").show
+------------------+
|current_database()|
+------------------+
|           default|
+------------------+
```

Author: Weiqing Yang <[email protected]>

Closes #15506 from weiqingy/prettyName.

(cherry picked from commit 56b0f5f)
Signed-off-by: Reynold Xin <[email protected]>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/misc.scala

@@ -564,4 +564,5 @@ case class CurrentDatabase() extends LeafExpression with Unevaluable {
   override def dataType: DataType = StringType
   override def foldable: Boolean = true
   override def nullable: Boolean = false
+  override def prettyName: String = "current_database"
 }