Bump tinymce from 5.7.1 to 5.9.0 in /samples/react-tinymce

Bumps [tinymce](https://github.com/tinymce/tinymce/tree/HEAD/modules/tinymce) from 5.7.1 to 5.9.0.
- [Release notes](https://github.com/tinymce/tinymce/releases)
- [Changelog](https://github.com/tinymce/tinymce/blob/develop/modules/tinymce/CHANGELOG.md)
- [Commits](https://github.com/tinymce/tinymce/commits/5.9.0/modules/tinymce)

---
updated-dependencies:
- dependency-name: tinymce
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

samples/react-tinymce/package-lock.json

@@ -15830,9 +15830,9 @@
       }
     },
     "tinymce": {
-      "version": "5.7.1",
-      "resolved": "https://registry.npmjs.org/tinymce/-/tinymce-5.7.1.tgz",
-      "integrity": "sha512-1gY8RClc734srSlkYwY0MQzmkS1j73PuPC+nYtNtrrQVPY9VNcZ4bOiRwzTbdjPPD8GOtv6BAk8Ww/H2RiqKpA=="
+      "version": "5.9.0",
+      "resolved": "https://registry.npmjs.org/tinymce/-/tinymce-5.9.0.tgz",
+      "integrity": "sha512-kaCTgR2NtJe9mgU46WPgUZFy5e2x9IgC9/kxqHg8ikFePi8bYE3bf3qmiWR+hXH/pr8PDFRsRCpqOuLEtdhG8Q=="
     },
     "tmp": {
       "version": "0.0.33",

samples/react-tinymce/package.json

@@ -22,7 +22,7 @@
     "react": "15.6.2",
     "react-dom": "15.6.2",
     "react-html-parser": "^2.0.2",
-    "tinymce": "^5.7.1"
+    "tinymce": "^5.9.0"
   },
   "devDependencies": {
     "@microsoft/sp-build-web": "~1.4.1",