use & to load completions for PowerShell

[hycinth22]

in src/building/suggested.md

You can use source ./src/etc/completions/x.py.<extension>
to load completions for your shell of choice,
or source .\src\etc\completions\x.py.ps1 for PowerShell.

to my best knowledge, there is no source command in powershell. it is available only in unix-like os.
we should use call operator(&) to load .ps1 script file for powershell

[hycinth22]

Other choice we may have:

1. Execute the filename directly: .\src\etc\completions\x.py.ps1
   This is equivalent to call operator, but its semantics are not clear enough

2. Invoke-Expression: Invoke-Expression -Command ".\src\etc\completions\x.py.ps1"
   too long and no obvious benefit

3. Dot-Sourcing: . .\src\etc\completions\x.py.ps1
   all definitions in the script will be import to the current session. we dont need so much