typo fix

When `sum(n.tspanner) != nrow(data)`, if `n.rgroup` isn't given we get this uninformative error below.

```r
library(Gmisc)

htmlTable(mtcars, n.tspanner = c(1, 32), tspanner = c('one','two'))
```
```
Error in sprintf("Your rows don't match in the n.tspanner, i.e. %d != %d",  : 
   argument "n.rgroup" is missing, with no default 
```
After the change:             
```r
htmlTable(mtcars, n.tspanner = c(1, 32), tspanner = c('one','two'))
```
```
Error in htmlTable(mtcars, n.tspanner = c(1, 32), tspanner = c("one",  : 
  Your rows don't match in the n.tspanner, i.e. 33 != 32
```