Incorrect statement in BigInteger.Pow(BigInteger, Int32) remarks (#11815)

[kanilsz]

Summary

Fix incorrect statement in BigInteger.Pow(BigInteger, Int32) remarks

Fixes dotnet/dotnet-api-docs/issues/#11815

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-numerics

[kanilsz]

Please add hacktoberfest-accepted label in case of a valid contribution.

[tannergooding]

@kanilsz, no "hacktoberfest-accepted" label has been required for Hacktoberfest for many years now.

The repo simply has to be participating by having the hacktoberfest topic (as this repo does) and your PR must not be explicitly invalidated (no spam, invalid, or similar label). A PR to a repo that isn't normally contributing can still specially accept a PR for hacktoberfest by applying the hacktoberfest-accepted label, but that's meant to be the exception not the norm (https://hacktoberfest.com/participation/).