Bump tinymce from 4.9.11 to 5.7.1 in /samples/react-tinymce

Bumps [tinymce](https://github.com/tinymce/tinymce-dist) from 4.9.11 to 5.7.1.
- [Release notes](https://github.com/tinymce/tinymce-dist/releases)
- [Changelog](https://github.com/tinymce/tinymce-dist/blob/master/CHANGELOG.md)
- [Commits](https://github.com/tinymce/tinymce-dist/compare/4.9.11...5.7.1)

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

samples/react-tinymce/package-lock.json

@@ -15830,9 +15830,9 @@
       }
     },
     "tinymce": {
-      "version": "4.9.11",
-      "resolved": "https://registry.npmjs.org/tinymce/-/tinymce-4.9.11.tgz",
-      "integrity": "sha512-nkSLsax+VY5DBRjMFnHFqPwTnlLEGHCco82FwJF2JNH6W+5/ClvNC1P4uhD5lXPDNiDykSHR0XJdEh7w/ICHzA=="
+      "version": "5.7.1",
+      "resolved": "https://registry.npmjs.org/tinymce/-/tinymce-5.7.1.tgz",
+      "integrity": "sha512-1gY8RClc734srSlkYwY0MQzmkS1j73PuPC+nYtNtrrQVPY9VNcZ4bOiRwzTbdjPPD8GOtv6BAk8Ww/H2RiqKpA=="
     },
     "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": "^4.7.6"
+    "tinymce": "^5.7.1"
   },
   "devDependencies": {
     "@microsoft/sp-build-web": "~1.4.1",