[linear_equations]Update the example environment

[longye-tian]

Dear John @jstac and Matt @mmcky ,

This pull request is to update the example environment in linear_equations.md.

I added the example environment to the place where we said 'for example' and followed up with a specific example.

I also checked the spelling/grammar for this lecture. #532

What do you think about the way I added example environment? Would you like to change or add something?

If you think this is a good way to add the example environment, I will follow this method for the rest part of the lecture series and check spelling and typos alongside.

Best ❤️
Longye

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[jstac]

Thanks @longye-tian , I think this looks great.

@mmcky ? I'll let you take it from here.

[mmcky]

Thanks @longye-tian this is looking great. Just made a comment on the labels.

[mmcky]

thanks @longye-tian