Update link to official Tcl website

Author: xdelaruelle

docs/installation.md

@@ -369,7 +369,7 @@ Supported module tools:
 
 Additional notes:
 
-* Tcl(/C) environment-modules requires [Tcl](https://www.tcl.tk/) to be
+* Tcl(/C) environment-modules requires [Tcl](https://www.tcl-lang.org/) to be
   installed (with header files and development libraries)
 * Lmod requires [Lua](https://www.lua.org/) and a couple of non-standard Lua libraries
   (`lua-posix`, `lua-filesystem`) to be available

docs/installing-environment-modules-without-root-permissions.md

@@ -6,7 +6,7 @@ Mac OS X system, together with Tcl on which it depends.
 
 ## Tcl
 
-1. Go to <https://www.tcl.tk> and download the latest Tcl sources. At
+1. Go to <https://www.tcl-lang.org> and download the latest Tcl sources. At
     the time of writing, the latest available Tcl version was 8.5.15,
     which can be downloaded from
     [here](https://prdownloads.sourceforge.net/tcl/tcl8.5.15-src.tar.gz).