A few minor updates to fix NOTEs from R CMD check

[jdblischak]

Yesterday I noticed a few NOTEs from R CMD check

In R-Devel, there is a new NOTE about missing package anchors:

* checking Rd cross-references ... NOTE
Found the following Rd file(s) with Rd \link{} targets missing package
anchors:
  rmst.Rd: Surv
Please provide package anchors for all Rd \link{} targets not in the
package itself and the base packages.

Since we are using the roxygen2 markdown syntax, our only option to add the package anchor is [survival::Surv()], which is awkward for the sentence, so I rearranged the link (in direct Rd syntax we could have used \code{\link[survival]{Surv}} to add the package anchor while still only displaying Surv).

I also updated the global variables that were renamed recently in #267 to fix the following NOTE:

Undefined global functions or variables:
  event_total event_trt n_risk_total

[jdblischak]

Confirmed no more NOTEs from the R-Devel R CMD check

[LittleBeannie]

Thanks you, @jdblischak, for cleaning the notes!