[SPARK-35171][R] Declare the markdown package as a dependency of the …

…SparkR package

### What changes were proposed in this pull request?
Declare the markdown package as a dependency of the SparkR package

### Why are the changes needed?
If we didn't install pandoc locally, running make-distribution.sh will fail with the following message:
```
— re-building ‘sparkr-vignettes.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
Pandoc (>= 1.12.3) not available. Falling back to R Markdown v1.
Error: processing vignette 'sparkr-vignettes.Rmd' failed with diagnostics:
The 'markdown' package should be declared as a dependency of the 'SparkR' package (e.g., in the 'Suggests' field of DESCRIPTION), because the latter contains vignette(s) built with the 'markdown' package. Please see yihui/knitr#1864 for more information.
— failed re-building ‘sparkr-vignettes.Rmd’
```

### Does this PR introduce _any_ user-facing change?
Yes. Workaround for R packaging.

### How was this patch tested?
Manually test. After the fix, the command `sh dev/make-distribution.sh -Psparkr -Phadoop-2.7 -Phive -Phive-thriftserver -Pyarn` in the environment without pandoc will pass.

Closes apache#32270 from xuanyuanking/SPARK-35171.

Authored-by: Yuanjian Li <yuanjian.li@databricks.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

R/pkg/DESCRIPTION

@@ -20,6 +20,7 @@ Depends:
 Suggests:
     knitr,
     rmarkdown,
+    markdown,
     testthat,
     e1071,
     survival,