Fix The Bug Brought by #212

[equt]

In PR #212, there is a bug that Fixedgq is used along with the formatprg option. According to the Vim/NeoVim doc

'formatprg' 'fp'	string (default "")
			global or local to buffer |global-local|

	The name of an external program that will be used to format the lines
	selected with the |gq| operator.  The program must take the input on
	stdin and produce the output on stdout.  The Unix program "fmt" is
	such a program.

this function should only be used in the formatexpr option.

c.c. #209