fix: Add space escaping for source command on makeprg setting

[yahorni]

Problem

A path to my ledger file contains spaces and during nvim/vim start I see the error:

$ nvim "ledger file with spaces.ledger"

$ vim "ledger file with spaces.ledger"

Tested both ledger and hledger binaries, only ledger is affected

Solution

Escaping source command argument with substitute (the same way it's done in #122) resolves the issue
Additionaly, I formatted the line with concatenation for better readability for both ledger/hledger

Tested on vim 9.1.785 and neovim v0.10.2
Probably fixes #144