[luatex] A small erratum in the manual

[Takuto ASAKURA]

Dear The LuaTeX Team,

in the latest "LuaTeX Reference Manual" (November 2019, Version 1.11),
at Section 13.2.3 (page 258) it has lines:

	n = getnofpages(<pdfe document>)
	bytes, waste = getnofpages(<pdfe document>)

I guess the second line in the above is actually explaining the function
`getmemoryusage()`, not `getnofpages()`.

Please kindly check it.

Best regards,
Takuto