Update documentation of rrecsys w.r.t. mostpopular

[albertiniufu]

In documentation of rrecsys, the configuration of alg mentions alg = "mostpopular".
However, in source code is just alg = "Popular".

[albertiniufu]

This issue causes another: evalPred with mostpopular does not work.
For example:

pop <-    evalPred(e, "Popular")
Train set 1 / 2 created with  134 users,  28 items and  1290  ratings.
Error in (function (classes, fdef, mtable)  : 
  unable to find an inherited method for function ‘predict’ for signature ‘"PPLclass"’

[UxxUnet]

It seems that mostpopular method is only desined for recommendation, not prediction.