Set savings goal and expense limit command descriptions have missing parameter prefixes

[wltan]

Leftover from #197

Despite being given as an example, set-savings-goal 500 does not actually work:

Invalid command format! 
set-savings-goal: Sets the monthly savings goal. Parameters: AMOUNT
Example: set-savings-goal 500

It should be a/500 instead of just 500, and Parameters: a/AMOUNT instead of just AMOUNT.

For reference, compare this to, say, the addi command:

Invalid command format! 
add-income: Adds an income to the finance tracker. Parameters: t/TITLE a/AMOUNT d/DATE [c/CATEGORY]...
Example: add-income t/Internship a/560 d/03/10/2020 c/Work

The same problem can be found for set-expense-limit:

Invalid command format! 
set-expense-limit: Sets the monthly expense limit. Parameters: AMOUNT
Example: set-expense-limit 500

The user guide has already been updated, it's just the in-app message that needs to be changed.

[wltan]

My bad, was already fixed in #209