Update install.md instructions regarding symlinks (#7231)

* Update install.md

Fixes `ln` command line bug that could hit users moving from packaged to source builds.

* Remove extra 'how to' command example

book/src/install.md

@@ -204,9 +204,11 @@ HELIX_RUNTIME=/home/user-name/src/helix/runtime
 Or, create a symlink in `~/.config/helix` that links to the source code directory:
 
 ```sh
-ln -s $PWD/runtime ~/.config/helix/runtime
+ln -Ts $PWD/runtime ~/.config/helix/runtime
 ```
 
+If the above command fails to create a symbolic link because the file exists either move `~/.config/helix/runtime` to a new location or delete it, then run the symlink command above again. 
+
 #### Windows
 
 Either set the `HELIX_RUNTIME` environment variable to point to the runtime files using the Windows setting (search for