Fix Scaladoc warning

Author: jyemin

driver-scala/src/main/scala/org/mongodb/scala/AggregateObservable.scala

@@ -124,7 +124,7 @@ case class AggregateObservable[TResult](private val wrapped: AggregatePublisher[
    * For MongoDB 5.0+, the aggregate command accepts a "let" option. This option is a document consisting of zero or more
    * fields representing variables that are accessible to the aggregation pipeline.  The key is the name of the variable and the value is
    * a constant in the aggregate expression language. Each parameter name is then usable to access the value of the corresponding
-   * expression with the "$$" syntax within aggregate expression contexts which may require the use of $expr or a pipeline.
+   * expression with the "$$" syntax within aggregate expression contexts which may require the use of '\$expr' or a pipeline.
    *
    * @param variables the variables
    * @return this