PR target/91276 - Doc typos in __builtin_crypto_vpmsum*

gcc/ChangeLog:

2020-02-26  Carl Love  <cel@us.ibm.com>

	PR target/91276
	* doc/extend.texi (PowerPC AltiVec Built-in Functions available on
	ISA 3.0): The builtin-function name __builtin_crypto_vpmsumb is only
	for the vector unsigned short arguments.  It is also listed as the
	name of the built-in for arguments vector unsigned short,
	vector unsigned int and vector unsigned long long built-ins.  The
	name of the builtins for these arguments should be:
	__builtin_crypto_vpmsumh, __builtin_crypto_vpmsumw and
	__builtin_crypto_vpmsumd respectively.

Signed-off-by: Carl Love <carll@us.ibm.com>