Fix missing $ sign in bash completion (#472)

Related issue  #64

Co-authored-by: Max Hacker <maximeh@google.com>

Author: maximehk

fire/completion.py

@@ -104,7 +104,7 @@ def _BashScript(name, commands, default_options=None):
 option_already_entered()
 {{
   local opt
-  for opt in ${{COMP_WORDS[@]:0:COMP_CWORD}}
+  for opt in ${{COMP_WORDS[@]:0:$COMP_CWORD}}
   do
     if [ $1 == $opt ]; then
       return 0