Some documentation pages still use old `workers` parameter

[HDash]

Replace worker parameter references with BPPARAM.

MotifPeeker/vignettes/MotifPeeker.Rmd

Line 208 in df701be

BPPARAM = BiocParallel::SerialParam(), # Use two CPU cores on a 16GB RAM machine

MotifPeeker/README.Rmd

Line 136 in df701be

workers = 2,