upgrade helm-docs

[bwagner5]

1. Issue, if available:
N/A

2. Description of changes:

• helm-docs v1.7.0 causes a checksum mismatch when using GOPROXY=direct, v1.8.1 is a stable release that matches the checksum
• helm-docs v1.7.0 depends on golang.org/x/sys/unix@20190215142949 which doesn't work with go 1.18 on darwin

$ go verison 
go version go1.17.8 darwin/amd64

$ GOPROXY=direct go install github.com/norwoodj/helm-docs/cmd/[email protected]
 # golang.org/x/sys/unix
../../go/pkg/mod/golang.org/x/[email protected]/unix/zsyscall_darwin_amd64.go:28:3: //go:linkname must refer to declared function or variable
../../go/pkg/mod/golang.org/x/[email protected]/unix/zsyscall_darwin_amd64.go:43:3: //go:linkname must refer to declared function or variable
../../go/pkg/mod/golang.org/x/[email protected]/unix/zsyscall_darwin_amd64.go:59:3: //go:linkname must refer to declared function or variable
../../go/pkg/mod/golang.org/x/[email protected]/unix/zsyscall_darwin_amd64.go:75:3: //go:linkname must refer to declared function or variable
../../go/pkg/mod/golang.org/x/[email protected]/unix/zsyscall_darwin_amd64.go:90:3: //go:linkname must refer to declared function or variable
../../go/pkg/mod/golang.org/x/[email protected]/unix/zsyscall_darwin_amd64.go:105:3: //go:linkname must refer to declared function or variable
../../go/pkg/mod/golang.org/x/[email protected]/unix/zsyscall_darwin_amd64.go:121:3: //go:linkname must refer to declared function or variable
../../go/pkg/mod/golang.org/x/[email protected]/unix/zsyscall_darwin_amd64.go:136:3: //go:linkname must refer to declared function or variable
../../go/pkg/mod/golang.org/x/[email protected]/unix/zsyscall_darwin_amd64.go:151:3: //go:linkname must refer to declared function or variable
../../go/pkg/mod/golang.org/x/[email protected]/unix/zsyscall_darwin_amd64.go:166:3: //go:linkname must refer to declared function or variable
../../go/pkg/mod/golang.org/x/[email protected]/unix/zsyscall_darwin_amd64.go:166:3: too many errors

3. How was this change tested?

$ go version
go version go1.18 darwin/amd64

$ GOPROXY=direct go install github.com/norwoodj/helm-docs/cmd/[email protected]
$ echo $?
0

4. Does this change impact docs?

• Yes, PR includes docs updates
• Yes, issue opened: link to issue
• No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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[njtran]

lgtm