fix: nat-gateway singular workflow name (#1619)

## Description

<!--
>Thank you for your contribution !
> Please include a summary of the change and which issue is fixed.
> Please also include the context.
> List any dependencies that are required for this change.

Fixes #123
Fixes #456
Closes #123
Closes #456
-->

## Pipeline Reference

<!-- Insert your Pipeline Status Badge below -->

| Pipeline |
| -------- |
|
[![avm.res.network.nat-gateway](https://github.com/eriqua/bicep-registry-modules/actions/workflows/avm.res.network.nat-gateway.yml/badge.svg?branch=fix%2Fworkflow-name)](https://github.com/eriqua/bicep-registry-modules/actions/workflows/avm.res.network.nat-gateway.yml)
|

## Type of Change

<!-- Use the check-boxes [x] on the options that are relevant. -->

- [x] Update to CI Environment or utlities (Non-module effecting
changes)
- [ ] Azure Verified Module updates:
- [ ] Bugfix containing backwards compatible bug fixes, and I have NOT
bumped the MAJOR or MINOR version in `version.json`:
- [ ] Someone has opened a bug report issue, and I have included "Closes
#{bug_report_issue_number}" in the PR description.
- [ ] The bug was found by the module author, and no one has opened an
issue to report it yet.
- [ ] Feature update backwards compatible feature updates, and I have
bumped the MINOR version in `version.json`.
- [ ] Breaking changes and I have bumped the MAJOR version in
`version.json`.
  - [ ] Update to documentation

## Checklist

- [x] I'm sure there are no other open Pull Requests for the same
update/change
- [ ] I have run `Set-AVMModule` locally to generate the supporting
module files.
- [x] My corresponding pipelines / checks run clean and green without
any errors or warnings

<!-- Please keep up to day with the contribution guide at
https://aka.ms/avm/contribute/bicep -->

.github/workflows/avm.res.network.nat-gateway.yml

@@ -1,4 +1,4 @@
-name: "avm.res.network.nat-gateways"
+name: "avm.res.network.nat-gateway"
 
 on:
   schedule: