docs($q): add missing closing parentheses in code example

Closes #4900
docs(guide/concepts): improve sentence wording

Before:

> Let's add some more logic to the example to
allow to enter and calculate the costs in different currencies and also pay the invoice.

After:

> Let's add some more logic to the example that
allows us to enter and calculate the costs in different currencies and also pay the invoice.

Closes #4903