Fix typo in configure.ac

[wegank]

Motivation

Hi! First PR here.

I think there's a typo in #9611:

nix/configure.ac

Lines 157 to 164 in ff6de4a

	# documentation generation switch
	AC_ARG_ENABLE(doc-gen, AS_HELP_STRING([--disable-doc-gen],[disable documentation generation]),
	ENABLE_DOC_GEN=$enableval, ENABLE_DOC_GEN=$ENABLE_BUILD)
	AC_SUBST(ENABLE_DOC_GEN)
	AS_IF(
	[test "$ENABLE_BUILD" == "no" && test "$ENABLE_GENERATED_DOCS" == "yes"],
	[AC_MSG_ERROR([Cannot enable generated docs when building overall is disabled. Please do not pass '--enable-doc-gen' or do not pass '--disable-build'.])])

so I'd like to fix it.

Context

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[Ericson2314]

Thank you!