build(deps): bump github.com/hashicorp/terraform-plugin-docs in /tools

Bumps [github.com/hashicorp/terraform-plugin-docs](https://github.com/hashicorp/terraform-plugin-docs) from 0.5.1 to 0.6.0.
- [Release notes](https://github.com/hashicorp/terraform-plugin-docs/releases)
- [Changelog](https://github.com/hashicorp/terraform-plugin-docs/blob/main/CHANGELOG.md)
- [Commits](hashicorp/terraform-plugin-docs@v0.5.1...v0.6.0)

---
updated-dependencies:
- dependency-name: github.com/hashicorp/terraform-plugin-docs
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

tools/go.mod

@@ -5,7 +5,7 @@ go 1.16
 require (
 	github.com/bflad/tfproviderlint v0.28.1
 	github.com/golangci/golangci-lint v1.44.2
-	github.com/hashicorp/terraform-plugin-docs v0.5.1
+	github.com/hashicorp/terraform-plugin-docs v0.6.0
 	golang.org/x/tools v0.1.9
 	mvdan.cc/sh/v3 v3.4.3
 )