[Rd] Wrong function name in R-ext manual (PR#10986)

[nsoranzo at tiscali.it]

Full_Name: Nicola Soranzo
Version: 2.6.2
OS: 
Submission from: (NULL) (147.122.2.166)


Section 8.1.5 "Threading issues" of "Writing R Extensions" says:

"These will be set when R_initialize_R is called, to values appropriate to the
main thread.
Stack-checking can be disabled by seting R_CStackLimit = (uintptr_t)-1, but it
is better to if possible set appropriate values."

R_initialize_R should be changed to Rf_initialize_R

Also I would modify the second sentence to "..., but it is better to set
appropriate values, if possible."