[MINOR][SPARKR] Add sparkr-vignettes.html to gitignore.

## What changes were proposed in this pull request?
Add ```sparkr-vignettes.html``` to ```.gitignore```.

## How was this patch tested?
No need test.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #15215 from yanboliang/ignore.

.gitignore

@@ -24,6 +24,7 @@
 R-unit-tests.log
 R/unit-tests.out
 R/cran-check.out
+R/pkg/vignettes/sparkr-vignettes.html
 build/*.jar
 build/apache-maven*
 build/scala*