fix: NaN result on toggle negative/positive number

[Dimas-Saputra-Me]

Fix: Issue #3

So if we trying to do an equation like 8 - -8 or 8 x -2 x -2 we will get NaN as a result display. I have made a few changes inside if (text === "+/-") function to fix the issue.

Here is the output before fix (stated in issue #3):

Here is the output after the fix:

Let me know if there is a feedback or I need to do something on this. Thanks in advance