Update model-basics.Rmd (#765)

Added second colon to 'MASS:rlm()' -> 'MASS::rlm()'

model-basics.Rmd

@@ -670,7 +670,7 @@ This chapter has focussed exclusively on the class of linear models, which assum
   between the parameter vector and the origin). This tends to make
   models that generalise better to new datasets from the same population.
 
-* __Robust linear models__, e.g. `MASS:rlm()`, tweak the distance to downweight 
+* __Robust linear models__, e.g. `MASS::rlm()`, tweak the distance to downweight 
   points that are very far away. This makes them less sensitive to the presence
   of outliers, at the cost of being not quite as good when there are no 
   outliers.