[SPARK-11409][SPARKR] Enable url link in R doc for Persist

[felixcheung]

Quick one line doc fix
link is not clickable

@shivaram

[shivaram]

LGTM. Thanks @felixcheung

[SparkQA]

Test build #44641 has finished for PR 9363 at commit 808cbac.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * abstract class CentralMomentAgg(child: Expression) extends ImperativeAggregate with Serializable\n * case class Variance(child: Expression,\n * case class VarianceSamp(child: Expression,\n * case class VariancePop(child: Expression,\n * case class Skewness(child: Expression,\n * case class Kurtosis(child: Expression,\n * case class Kurtosis(child: Expression) extends UnaryExpression with AggregateExpression1\n * case class Skewness(child: Expression) extends UnaryExpression with AggregateExpression1\n * case class Variance(child: Expression) extends UnaryExpression with AggregateExpression1\n * case class VariancePop(child: Expression) extends UnaryExpression with AggregateExpression1\n * case class VarianceSamp(child: Expression) extends UnaryExpression with AggregateExpression1\n