[BUGFIX] Use consistent variable name in FlexFormProcessor example

[ghermens]

Followup to PR #4574

[github-actions[bot]]

The backport to 11.5 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

``` error: could not apply 0e6cc81a... [BUGFIX] Use consistent variable name in FlexFormProcessor example hint: After resolving the conflicts, mark them with hint: "git add/rm ", then run hint: "git cherry-pick --continue". hint: You can instead skip this commit with "git cherry-pick --skip". hint: To abort and get back to the state before "git cherry-pick", hint: run "git cherry-pick --abort". hint: Disable this message with "git config advice.mergeConflict false" ```

stdout

``` Auto-merging Documentation/ApiOverview/FlexForms/Index.rst CONFLICT (content): Merge conflict in Documentation/ApiOverview/FlexForms/Index.rst ```

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-11.5 11.5
# Navigate to the new working tree
cd .worktrees/backport-11.5
# Create a new branch
git switch --create backport-4575-to-11.5
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 0e6cc81ab5e014c0e16ad3799032cd6988263607
# Push it to GitHub
git push --set-upstream origin backport-4575-to-11.5
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-11.5

Then, create a pull request where the base branch is 11.5 and the compare/head branch is backport-4575-to-11.5.

[ghermens]

Backport to 11.5 is not necessary: The FlexFormProcessor was introduced in 12 ...