fix back end spelling

[mairaw]

I noticed in the docs that back end was misspelled. It's either two words or hyphened when used as an adjective: https://docs.microsoft.com/en-us/style-guide/a-z-word-list-term-collections/b/back-end

I've only changed text and comments, but not variable names.

[imback82]

Can't backend,back-end and back end be used interchangeably? https://en.wikipedia.org/wiki/Back_end

[mairaw]

I pointed to the Microsoft style guide above - that's our official guideline. Also, if you look at the examples under computing in the Wikipedia article, they follow the same rules as I described. But it's up to you whether you wanna take this change or not.

[mairaw]

I rolled back some of the changes and kept only the ones in code comments and Markdown.