bash-completion: curly brace missed for findmnt

bash-completion: curly brace missed for findmnt

This patch provides fix for bash-completion/fndmt script. There
is curly brace missed in the completion generation for the -M/--mountpoint
option.

In other way we will get following messages:

bash: /etc/bash_completion.d/findmnt: line 91: unexpected EOF while looking for matching `)'
bash: /etc/bash_completion.d/findmnt: line 141: syntax error: unexpected end of file

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>