Fix the definition of `real2complex` for spherical harmonics #685

Job	Run time
Lint Rust code (stable, x86_64-unknown-linux-gnu)	6s
check examples / benchmarks	1m 26s
ubuntu-22.04 / rust stable / static C library	5m 23s
ubuntu-22.04 / rust stable / release valgrind	9m 18s
ubuntu-22.04 / rust 1.74 / cmake 3.16	6m 41s
macos-14 / rust stable	4m 5s
windows-2019 / rust stable	9m 55s
	36m 54s

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