Fix strict settings (#25)

It looks like these scripts were trying to use strict Javascript
parsing, but had a typo in the directive. (Or maybe this is a
different directive that I don't know about, I admit I do very little
Javascript development. If that is the case, please just ignore this
pull request. But I could not find any suggestion that this would do
anything.)

Also, `index.js` was missing the directive entirely, and I though you
might want it there as well?

src/delete.js

@@ -1,4 +1,4 @@
-'use sstrict'
+'use strict'
 const Core = require('@actions/core')
 
 module.exports = async (octokit, context) => {

src/download.js

@@ -1,4 +1,4 @@
-'use sstrict'
+'use strict'
 const Core = require('@actions/core')
 const FS = require('fs');
 const Path = require('path');

src/index.js

@@ -1,3 +1,4 @@
+'use strict'
 const github = require('@actions/github')
 const Core = require('@actions/core')
 

src/update.js

@@ -1,10 +1,10 @@
-'use sstrict'
+'use strict'
 const FS = require('fs')
 const Core = require('@actions/core')
 const Mime = require('mime-types')
 const Path = require('path');
 const {
-    findRelease, 
+    findRelease,
     splitAssetsString
 } = require('./utils')
 

src/utils.js

@@ -1,4 +1,4 @@
-'use sstrict'
+'use strict'
 
 const splitAssetsString = assetStr => assetStr.split(';').filter(a => a)