minor #1974 Update index.rst (elkuku)

This PR was merged into the 2.x branch.

Discussion
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Update index.rst

Fix an undefined variable

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  |no <!-- please update src/**/CHANGELOG.md files -->
| Issues        | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT

There seems to be an undefined variable in the example

Commits
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fad816d Update index.rst

Author: javiereguiluz

src/React/doc/index.rst

@@ -82,7 +82,7 @@ For example:
     {% extends 'base.html.twig' %}
 
     {% block body %}
-        <div {{ react_component('Hello', { 'fullName': number }) }}>
+        <div {{ react_component('Hello', { fullName: 'Fabien' }) }}>
             Loading... <i class="fas fa-cog fa-spin fa-3x"></i>
         </div>