Fix an issue in Emscripten README

[StanFromIreland]

Looking at:

cpython/configure.ac

Lines 7796 to 7804 in 9e23652

	# Check whether to disable test modules. Once set, setup.py will not build
	# test extension modules and "make install" will not install test suites.
	AC_MSG_CHECKING([for --disable-test-modules])
	AC_ARG_ENABLE([test-modules],
	[AS_HELP_STRING([--disable-test-modules], [don't build nor install test modules])], [
	AS_VAR_IF([enable_test_modules], [yes], [TEST_MODULES=yes], [TEST_MODULES=no])
	], [TEST_MODULES=yes])
	AC_MSG_RESULT([$TEST_MODULES])
	AC_SUBST([TEST_MODULES])