Fix typo in completions help (#431)

Author: teeberg

news/431.docs.md

@@ -0,0 +1 @@
+Fixed typo in docs about setting up shell completion

src/cleo/commands/completions_command.py

@@ -87,7 +87,7 @@ class CompletionsCommand(Command):
 generated script to one of those directories, or add your own \
 to this list.
 
-Adding a custom directory is often the safest best if you're unsure \
+Adding a custom directory is often the safest bet if you're unsure \
 of which directory to use. First create the directory, for this \
 example we'll create a hidden directory inside our `<options=bold>$HOME</>` directory