Docs: Update SQL example for current_time() and current_date().

[codetyri0n]

Which issue does this PR close?

• Closes Update documentation for current_date and current_time functions #18199

What changes are included in this PR?

• Added a SQL example in scalar functions md to demonstrate setting execution time zone (optional) for current_time() and current_date().

Are these changes tested?

Are there any user-facing changes?

[codetyri0n]

@alamb could you please take a look

[Jefffrey]

I believe these should be modified in the source files instead of this doc as it gets autogenerated, see:

datafusion/datafusion/functions/src/datetime/current_date.rs

Lines 32 to 40 in b5b7f9b

	#[user_doc(
	doc_section(label = "Time and Date Functions"),
	description = r#"
	Returns the current date in the session time zone.
	The `current_date()` return value is determined at query time and will return the same date, no matter when in the query plan the function executes.
	"#,
	syntax_example = "current_date()"
	)]

You could add it via a sql_example block like here for example:

datafusion/datafusion/functions-nested/src/range.rs

Lines 63 to 77 in b5b7f9b

	sql_example = r#"```sql
	> select range(2, 10, 3);
	+-----------------------------------+
	| range(Int64(2),Int64(10),Int64(3))|
	+-----------------------------------+
	| [2, 5, 8] |
	+-----------------------------------+
	> select range(DATE '1992-09-01', DATE '1993-03-01', INTERVAL '1' MONTH);
	+--------------------------------------------------------------+
	| range(DATE '1992-09-01', DATE '1993-03-01', INTERVAL '1' MONTH) |
	+--------------------------------------------------------------+
	| [1992-09-01, 1992-10-01, 1992-11-01, 1992-12-01, 1993-01-01, 1993-02-01] |
	+--------------------------------------------------------------+
	```"#,

Then run ./dev/update_function_docs.sh to generate that doc

[codetyri0n]

thanks, ill modify this accordingly

[alamb]

Thanks @Jefffrey and @codetyri0n - this looks like an improvement to me