[SPARKR][MINOR] Fix LDA doc

## What changes were proposed in this pull request?

This PR tries to fix the name of the `SparkDataFrame` used in the example. Also, it gives a reference url of an example data file so that users can play with.

## How was this patch tested?

Manual test.

Author: Junyang Qian <junyangq@databricks.com>

Closes #14853 from junyangq/SPARKR-FixLDADoc.

R/pkg/R/mllib.R

@@ -994,18 +994,22 @@ setMethod("spark.survreg", signature(data = "SparkDataFrame", formula = "formula
 #' @export
 #' @examples
 #' \dontrun{
-#' text <- read.df("path/to/data", source = "libsvm")
+#' # nolint start
+#' # An example "path/to/file" can be
+#' # paste0(Sys.getenv("SPARK_HOME"), "/data/mllib/sample_lda_libsvm_data.txt")
+#' # nolint end
+#' text <- read.df("path/to/file", source = "libsvm")
 #' model <- spark.lda(data = text, optimizer = "em")
 #'
 #' # get a summary of the model
 #' summary(model)
 #'
 #' # compute posterior probabilities
-#' posterior <- spark.posterior(model, df)
+#' posterior <- spark.posterior(model, text)
 #' showDF(posterior)
 #'
 #' # compute perplexity
-#' perplexity <- spark.perplexity(model, df)
+#' perplexity <- spark.perplexity(model, text)
 #'
 #' # save and load the model
 #' path <- "path/to/model"