[Rd] minor correction to the r internals manual

[Wacek Kusnierczyk]

sec. 1.1 says:

"both types of node structure have as their first three fields a 32-bit
sxpinfo header and then three pointers [...]"

that's *four* fields, as seen in src/include/Rinternals.h:208+:

#define SEXPREC_HEADER \
    struct sxpinfo_struct sxpinfo; \
    struct SEXPREC *attrib; \
    struct SEXPREC *gengc_next_node, *gengc_prev_node

vQ