Fix "subscript out of bounds" error

From https://bioconductor.org/checkResults/devel/bioc-LATEST/derfinder/nebbiolo1-buildsrc.html

```
* creating vignettes ... ERROR
--- re-building ‘derfinder-quickstart.Rmd’ using rmarkdown

Quitting from lines 343-378 (derfinder-quickstart.Rmd)
Error: processing vignette 'derfinder-quickstart.Rmd' failed with diagnostics:
subscript out of bounds
--- failed re-building ‘derfinder-quickstart.Rmd’

--- re-building ‘derfinder-users-guide.Rmd’ using rmarkdown
Warning in in_dir(input_dir(), expr) :
  You changed the working directory to /tmp/RtmphAsNWX/Rbuild4c29c999c132/derfinder/vignettes/analysisResults (probably via setwd()). It will be restored to /tmp/RtmphAsNWX/Rbuild4c29c999c132/derfinder/vignettes. See the Note section in ?knitr::knit

Quitting from lines 1341-1459 (derfinder-users-guide.Rmd)
Error: processing vignette 'derfinder-users-guide.Rmd' failed with diagnostics:
subscript out of bounds
--- failed re-building ‘derfinder-users-guide.Rmd’

SUMMARY: processing the following files failed:
  ‘derfinder-quickstart.Rmd’ ‘derfinder-users-guide.Rmd’

Error: Vignette re-building failed.
Execution halted
```

It turns out that `citation("derfinderHelper")` has a length of 2. So it automatically
creates two entries in the `bib` object: `bib[['derfinderHelper1']]` and `bib[['derfinderHelper2']]`.
Thus `bib[['derfinderHelper']]` is not found later on, and this leads to the subscript out of bounds.

vignettes/derfinder-quickstart.Rmd

@@ -104,7 +104,7 @@ bib <- c(
         year = 2017, doi = "10.18129/B9.bioc.AnnotationDbi"
     ),
     BiocParallel = citation("BiocParallel"),
-    derfinderHelper = citation("derfinderHelper"),
+    derfinderHelper = citation("derfinderHelper")[1],
     GenomicAlignments = citation("GenomicAlignments"),
     GenomicFeatures = citation("GenomicFeatures"),
     GenomicFiles = citation("GenomicFiles"),

vignettes/derfinder-users-guide.Rmd

@@ -70,7 +70,7 @@ bib <- c(
         year = 2017, doi = "10.18129/B9.bioc.AnnotationDbi"
     ),
     BiocParallel = citation("BiocParallel"),
-    derfinderHelper = citation("derfinderHelper"),
+    derfinderHelper = citation("derfinderHelper")[1],
     GenomicAlignments = citation("GenomicAlignments"),
     GenomicFeatures = citation("GenomicFeatures"),
     GenomicFiles = citation("GenomicFiles"),