In which directory did you save the .ps1 file ?
You have to position yourself in the directory where you saved the .ps1 file.
PowerCLI C:\Program Files\VMware\Infrastructure\vSphere PowerCLI> cd c:\Scripts
or give the full path to the script
PowerCLI C:\Program Files\VMware\Infrastructure\vSphere PowerCLI> C:\Scripts\vm-audit.ps1