Fix typos in interest-is-interesting instructions (#608)

greater or equal than -> greater than or equal to

exercises/concept/interest-is-interesting/.docs/instructions.md

@@ -4,8 +4,8 @@ In this exercise you'll be working with savings accounts. Each year, the balance
 
 - -3.213% for a negative balance.
 - 0.5% for a positive balance less than `1000` dollars.
-- 1.621% for a positive balance greater or equal than `1000` dollars and less than `5000` dollars.
-- 2.475% for a positive balance greater or equal than `5000` dollars.
+- 1.621% for a positive balance greater than or equal to `1000` dollars and less than `5000` dollars.
+- 2.475% for a positive balance greater than or equal to `5000` dollars.
 
 Each year, **before** the balance of your savings account is updated, the government allows you to donate a percentage of your money to charity, tax free. Because you're a nice person, if your balance is positive at the end of the year, you donate twice this amount to charities, rounded down to the nearest whole dollar.