Fix error for env variables example in docker reference

Signed-off-by: Charles Law <claw@conduce.com>

docs/reference/builder.md

@@ -361,7 +361,7 @@ RUN /bin/bash -c 'source $HOME/.bashrc ; echo $HOME'
 > This means that normal shell processing does not happen. For example,
 > `RUN [ "echo", "$HOME" ]` will not do variable substitution on `$HOME`.
 > If you want shell processing then either use the *shell* form or execute
-> a shell directly, for example: `RUN [ "sh", "-c", "echo", "$HOME" ]`.
+> a shell directly, for example: `RUN [ "sh", "-c", "echo $HOME" ]`.
 
 The cache for `RUN` instructions isn't invalidated automatically during
 the next build. The cache for an instruction like