Vector addition incorrect information

In 7.7.1 it (incorrectly) states that one is "unable to change the number of elements in a vector". However, elements can be added to a vector with this syntax:

```r
# More commonly used syntax
a <- c(1,2,3)
a <- c(a, 4)

# Doesn't throw an error
a <- c(1,2,3)
a[4] <- 4
```

Should be clear _why_ the common syntax is more common (i.e., no need to know the current number of elements). 