Fix missing slash in app key allowed characters hint

[mayorova]

What this PR does / why we need it:

Making sure that the backslash \ appears in the list of the valid characters for the application key:

That character is supported according to

porta/app/models/application_key.rb

Lines 15 to 17 in 26743e8

	# A-Z a-z 0-9 ! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~
	# Spaces are not allowed
	validates :value, format: { with: /\A[\x21-\x7E]+\Z/ },

Which issue(s) this PR fixes

Verification steps

Special notes for your reviewer: