fix argument name for ipv6 masking

[danstadler-pdx]

running the code without this rename causes alloy to stop during config loading. Line 327 expects the argument name "mask_ipv6_value".

Tested the fix locally and it worked for me.

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:white_check_mark: danstadler-pdx
:x: bentonam
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[CLAassistant]

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