fix Windows short-circuit in man_pages.is_available

[HrachShah]

Summary

is_available() guarded against Windows with os.system == 'nt'. os.system is the built-in function object from the os module, never a string, so the comparison was always False and the function never short-circuited on Windows even though that was clearly the author's intent (the surrounding code is checking whether the running OS has the man binary).

On Windows the function fell through to subprocess.run([MAN_COMMAND, ...]), which is missing from PATH and raised FileNotFoundError that was then swallowed by the bare except Exception, so users saw a confusing 'no man pages found' instead of an immediate 'not on Windows' early return.

Fix

Replace os.system with os.name — the standard Python idiom for OS detection (returns 'nt' on Windows, 'posix' on Linux/macOS).

Testing

The change is one character; manual verification: os.system == 'nt' evaluates to False on every platform including Windows, while os.name == 'nt' correctly returns True on Windows and False on Linux/macOS.