Fix checkpoint example in "Writing custom actions"

The example was not using the correct parameters for setting up the checkpoint - You pass in an `opts` object rather than individual parameters.

Signed-off-by: jvmdc <109624469+jvmdc@users.noreply.github.com>

docs/features/software-templates/writing-custom-actions.md

@@ -263,7 +263,7 @@ Idempotent action could be achieved via the usage of checkpoints.
 Example:
 
 ```ts title="plugins/my-company-scaffolder-actions-plugin/src/vendor/my-custom-action.ts"
-const res = await ctx.checkpoint?.('create.projects', async () => {
+const res = await ctx.checkpoint?.({key: 'create.projects', fn: async () => {
   const projectStgId = createStagingProjectId();
   const projectProId = createProductionProjectId();
 
@@ -271,7 +271,7 @@ const res = await ctx.checkpoint?.('create.projects', async () => {
     projectStgId,
     projectProId,
   };
-});
+}});
 ```
 
 You have to define the unique key in scope of the scaffolder task for your checkpoint. During the execution task engine