Fix AnimationMixer docs example code for `get_root_motion_rotation_accumulator` #33863

Job	Run time
📊 Static checks / Code style, file formatting, and docs	36s
🍎 macOS / Editor (target=editor, tests=yes)	4m 13s
🍏 iOS / Template (target=template_release)	2m 2s
🤖 Android / Editor (target=editor)	4m 41s
🐧 Linux / Editor w/ Mono (target=editor)	4m 48s
🍎 macOS / Template (target=template_release)	3m 21s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	3m 37s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	4m 19s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	16m 6s
🌐 Web / Template w/o threads (target=template_release, threads=no)	4m 23s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	3m 21s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	7m 59s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	7m 55s
🐧 Linux / Template w/ Mono (target=template_release)	2m 24s
🏁 Windows / Editor (target=editor, tests=yes)	10m 50s
🐧 Linux / Minimal template (target=template_release, everything disabled)	2m 26s
🏁 Windows / Template (target=template_release)	8m 53s
🪲 Godot CPP / Build and test Godot CPP	7m 40s
	1h 39m 34s

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