Fix `os.sep` mention for macOS

Thanks @gvanem

https://github.com/swaroopch/byte-of-python/commit/8cad9e19d7900dee75ba1de82ea4ce35b5813878#commitcomment-22263931

problem_solving.md

@@ -44,7 +44,7 @@ You will notice how we have converted our *design* into *code* in a step-by-step
 
 We make use of the `os` and `time` modules by first importing them. Then, we specify the files and directories to be backed up in the `source` list. The target directory is where we store all the backup files and this is specified in the `target_dir` variable. The name of the zip archive that we are going to create is the current date and time which we generate using the `time.strftime()` function. It will also have the `.zip` extension and will be stored in the `target_dir` directory.
 
-Notice the use of the `os.sep` variable - this gives the directory separator according to your operating system i.e. it will be `'/'` in GNU/Linux and Unix, it will be `'\\'` in Windows and `':'` in Mac OS. Using `os.sep` instead of these characters directly will make our program portable and work across all of these systems.
+Notice the use of the `os.sep` variable - this gives the directory separator according to your operating system i.e. it will be `'/'` in GNU/Linux, Unix, macOS, and will be `'\\'` in Windows. Using `os.sep` instead of these characters directly will make our program portable and work across all of these systems.
 
 The `time.strftime()` function takes a specification such as the one we have used in the above program. The `%Y` specification will be replaced by the year with the century. The `%m` specification will be replaced by the month as a decimal number between `01` and `12` and so on. The complete list of such specifications can be found in the [Python Reference Manual](http://docs.python.org/3/library/time.html#time.strftime).