Remove backticks from -inject-labels flag desc

[colega]

Backticks in a flag description have a special meaning, in both standardflag and the pflag package used here.
Because of this, flag was described as:

--inject-labels tk prune       add tanka environment label to each created resource. Required for tk prune.

Now it will look like:

--inject-labels                add tanka environment label to each created resource. Required for 'tk prune'.

See spf13/pflag#200

Signed-off-by: Oleg Zaytsev mail@olegzaytsev.com

[julienduchesne]

Thanks