[Rd] typo in R-exts manual

[Duncan Murdoch]

Just noticed a typo (INIINITY instead of INFINITY) in WRE.  Here's a patch:

Index: doc/manual/R-exts.texi
===================================================================
--- doc/manual/R-exts.texi      (revision 89053)
+++ doc/manual/R-exts.texi      (working copy)
@@ -6596,7 +6596,7 @@
  about an empty argument list in C23 mode.)

  @item
- using code{INIINITY} and @code{NAN} are available via
+ using code{INFINITY} and @code{NAN} are available via
  header @file{float.h} and deprecated in @file{math.h}.

  @item