Add Errata for Section 12.6 Code Example

[mackersD]

This PR contains an addition to the errata markdown to include a correction to the code example in exponentscanf.s in section 12.6.

It was discovered that the fscanf function requires modifiers in the scanformat variable so that it stores the matched input values with a matching size to the memory locations.

As an example, if you were to run the code and enter "2 3" for 2^3 without the modifiers, and the stack location for the exponent location previously contained 0x0000000700000000, fscanf would populate this stack memory location with 0x0000000700000003. When getting to the exponent function, it would loop 30064771075 times versus 3 times.