[Rd] typos in src/main/gram.y (PR#8780)

[htang at hpl.hp.com]

In src/main/gram.y, the documentation for R_ParseVector has a wrong
signature:

SEXP R_ParseVector(TextBuffer *text, int n, ParseStatus *status)

should be

SEXP R_ParseVector(SEXP text, int n, ParseStatus *status)

In addition, the two occurrences of "IOBuffer" in the documentation
should be replaced by "IoBuffer".

version.string = Version 2.3.0 beta (2006-04-14 r37779)

Best,
Hsiu-Khuern Tang
HP Labs