Docs: Fix date function in spark procedures docs

[myandpr]

What problem is solved by this MR？Why this MR is needed?

It doesn`t work when I use 'date_sub(current_date(), 10)' in 'spark_catalog.system.expire_snapshots'.

because we use a parser extension, the procedures do not allow arbitrary expressions.

I think that we'd better modify the document first to prevent users from encountering errors.

environment:

Spark	Iceberg
3.0	0.12

scala> spark.sql("CALL spark_catalog.system.expire_snapshots('db.table', date_sub(current_date(), 10), 5)")

org.apache.spark.sql.catalyst.parser.extensions.IcebergParseException:
no viable alternative at input 'date_sub('(line 1, pos 81)

== SQL ==
CALL spark_catalog.system.expire_snapshots('db.table', date_sub(current_date(), 10), 5)
---------------------------------------------------------------------------------^^^

  at org.apache.spark.sql.catalyst.parser.extensions.IcebergParseException.withCommand(IcebergSparkSqlExtensionsParser.scala:294)
  at org.apache.spark.sql.catalyst.parser.extensions.IcebergSparkSqlExtensionsParser.parse(IcebergSparkSqlExtensionsParser.scala:158)
  at org.apache.spark.sql.catalyst.parser.extensions.IcebergSparkSqlExtensionsParser.parsePlan(IcebergSparkSqlExtensionsParser.scala:105)
  at org.apache.spark.sql.SparkSession.$anonfun$sql$2(SparkSession.scala:605)
  at org.apache.spark.sql.catalyst.QueryPlanningTracker.measurePhase(QueryPlanningTracker.scala:111)
  at org.apache.spark.sql.SparkSession.$anonfun$sql$1(SparkSession.scala:605)
  at org.apache.spark.sql.SparkSession.withActive(SparkSession.scala:764)
  at org.apache.spark.sql.SparkSession.sql(SparkSession.scala:602)
  ... 47 elided

How to solve this problem?

I think that we'd better modify the document first to prevent users from encountering errors.

Related issue

#2341

[myandpr]

@RussellSpitzer , could you help to review this PR? :)

[RussellSpitzer]

Thanks! I have been meaning to fix this for a while