Fix docstring for fmpz_poly_is_squarefree #2846

Job	Run time
Alpine Linux, musl, 32-bit (assert, x1.5)	13m 54s
macOS-M1, Clang (x3)	9m 33s
GCC build regression check (no tests)	1m 6s
macOS-x86 Clang with BLAS (x2)	18m 9s
MinGW GCC (x0.5)	10m 57s
MSVC (x1)	9m 1s
Nemo.jl (temporary branch)	13m 48s
Ubuntu Clang with examples (x5)	12m 36s
Ubuntu GCC, Code Coverage (x10)	30m 14s
Ubuntu GCC with NTL (assert, x2)	8m 28s
	2h 7m 46s

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