Documentation for asSparse parameter of colPair()

Currently the documentation for colPairs() states:
`When asSparse=TRUE, the return value will be a triplet-form sparse matrix where each row/column corresponds to a column of x.`

But running the documented example returns a a dgCMatrix:
```
> colPair(sce, "regulators", asSparse=TRUE)
100 x 100 sparse Matrix of class "dgCMatrix"
```