Advise overwrite rather than append to existing bash completion files

There should be no reason to append to existing ones. Other similar
completion file install recipes overwrite, too.

src/cli/help.rs

@@ -175,7 +175,7 @@ pub(crate) static COMPLETIONS_HELP: &str = r"Discussion:
     Run the command:
 
         $ mkdir -p ~/.local/share/bash-completion/completions
-        $ rustup completions bash >> ~/.local/share/bash-completion/completions/rustup
+        $ rustup completions bash > ~/.local/share/bash-completion/completions/rustup
 
     This installs the completion script. You may have to log out and
     log back in to your shell session for the changes to take effect.
@@ -270,7 +270,7 @@ pub(crate) static COMPLETIONS_HELP: &str = r"Discussion:
 
     Bash:
 
-        $ rustup completions bash cargo >> ~/.local/share/bash-completion/completions/cargo
+        $ rustup completions bash cargo > ~/.local/share/bash-completion/completions/cargo
 
     Zsh: