Update README.md

Use canonical link format for CRAN packages.

README.md

@@ -5,11 +5,11 @@ It implements the KHB method as described in [Karlson, Holm, and Breen (2012)](h
 
 # Dependencies
 The following packages not provided in the base installation of R are required:
-- [`systemfit`](https://cran.r-project.org/web/packages/systemfit/index.html)
-- [`sandwich`](https://cran.r-project.org/web/packages/sandwich/index.html)
+- [`systemfit`](https://CRAN.R-project.org/package=systemfit)
+- [`sandwich`]( https://CRAN.R-project.org/package=sandwich)
 
 # Installation
-The package can be installed using the [`devtools`](https://cran.r-project.org/web/packages/devtools/index.html) package with the following
+The package can be installed using the [`devtools`]( https://CRAN.R-project.org/package=devtools) package with the following
 commands:
 ```{r}
 install.packages("devtools")