Minor documentation improvements

[marlycormar]

Thanks for developing the NNG4 game!

I found a few minor documentation issues, which this PR addresses. I am not entirely sure which notation standard you would prefer throughout the game, so I made an informed choice by aligning the notation with the initial statement in each case. I also updated the corresponding files used for translating the documentation into other languages so that these changes remain consistent across the project.

As I continue working through the game, I would be happy to open additional PRs if I come across other minor issues.

Summary of changes

• Removed dangling end quotes from one_eq_succ_zero and updated the corresponding translation files.
• Removed dangling end quotes from + and updated the corresponding translation files.
• add_comm used both $a/b$ and $x/y$; I aligned the notation to use $a$ and $b$ throughout and updated the corresponding translation files.
• zero_add used both $n$ and $x$; I aligned the notation to use $n$ throughout and updated the corresponding translation files.
• zero_mul used both $m$ and $x$; I aligned the notation to use $m$ throughout and updated the corresponding translation files.

[joneugster]

Thank you!

I personally think the variable renaming is kind of superfluous and as inconsistent as it was before, but now it's here, let's just merge this as-is