Fix documented examples to not use the deprecated is_alphabetic function #1154

Job	Run time
Build documentation	56s
Bench	40s
Check formatting	4s
Test (stable)	31s
Test (beta)	39s
Coverage	55s
Test (nightly)	37s
Test minimal rust version	1m 58s
Test (1.65.0)	2m 29s
Test (stable, --features "std")	36s
Test (stable, --no-default-features)	52s
Test (stable, --no-default-features --features "alloc")	48s
Test (nightly, --no-default-features)	37s
Test (nightly, --no-default-features --features "alloc")	40s
	12m 22s

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