🩹 Fix Nextion cstr/fstr typo

[thisiskeithb]

Description

While running the build_all_examples script, I found that our Nextion TFT-based config would not build:

details:

Marlin/src/lcd/extui/nextion/nextion_extui.cpp: In function 'void ExtUI::onUserConfirmRequired(int, FSTR_P, FSTR_P)':
Marlin/src/lcd/extui/nextion/nextion_extui.cpp:64:27: error: 'cstr' was not declared in this scope
     onUserConfirmRequired(cstr);
                           ^~~~
Marlin/src/lcd/extui/nextion/nextion_extui.cpp:64:27: note: suggested alternative: 'fstr'
     onUserConfirmRequired(cstr);
                           ^~~~
                           fstr
*** [.pio/build/mega2560/src/src/lcd/extui/nextion/nextion_extui.cpp.o] Error 1
============================================================== [FAILED] Took 10.56 seconds ==============================================================

Environment    Status    Duration
-------------  --------  ------------
mega2560       FAILED    00:00:10.557
========================================================= 1 failed, 0 succeeded in 00:00:10.557 =========================================================
Failed

This appears to be an obvious typo & now matches the other extui changes made in #26539

Requirements

Any config with NEXTION_TFT.

Benefits

NEXTION_TFT-based configs will build.

Configurations

config/examples/Nextion

Related Issues

• Update ProUI Plot graph #26539
• Build All Configurations Status - May 2024 Configurations#1052