Small fixes to development_guide: title and label names
Small fixes to development_guide: title and label names

* Fix title to use a proper "title" section marker
* Fix labels by adding a " " after the ":"
* Fix OS labels after obvious mishap
* Sort labels

davidszotten/pytest,tomviner/pytest,txomon/pytest,tareqalayan/pytest,ddboline/pytest,The-Compiler/pytest,markshao/pytest,Akasurde/pytest,tomviner/pytest,nicoddemus/pytest,pfctdayelise/pytest,pytest-dev/pytest,The-Compiler/pytest,nicoddemus/pytest,hackebrot/pytest,alfredodeza/pytest,RonnyPfannschmidt/pytest,rmfitzpatrick/pytest