doc: simplify CRAN mirror text in benchmark guide

PR-URL: #39287
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/guides/writing-and-running-benchmarks.md

@@ -74,11 +74,8 @@ install.packages("ggplot2")
 install.packages("plyr")
 ```
 
-In the event that a message is reported stating that a CRAN mirror must be
-selected first, specify a mirror by adding in the repo parameter.
-
-If we used the "<http://cran.us.r-project.org>" mirror, it could look something
-like this:
+If a message states that a CRAN mirror must be selected first, specify a mirror
+with the `repo` parameter.
 
 ```r
 install.packages("ggplot2", repo="http://cran.us.r-project.org")