[MINOR][SPARKR] ignore Rplots.pdf test output after running R tests

## What changes were proposed in this pull request?

After running R tests in local build, it outputs Rplots.pdf. This one should be ignored in the git repository.

Author: wangmiao1981 <wm624@hotmail.com>

Closes #18518 from wangmiao1981/ignore.

.gitignore

@@ -25,6 +25,7 @@ R-unit-tests.log
 R/unit-tests.out
 R/cran-check.out
 R/pkg/vignettes/sparkr-vignettes.html
+R/pkg/tests/fulltests/Rplots.pdf
 build/*.jar
 build/apache-maven*
 build/scala*