[Rd] trivial typo in man/pretty.Rd

[Ben Bolker]

  patch against recent SVN ...
  as far as I can tell this trivial typo has been there for 20 years:

https://github.com/wch/r-source/blame/ba7920a99fb2fb62b89e404e65f8b132ed4c150a/src/library/base/man/pretty.Rd

===================================================================
--- pretty.Rd	(revision 74426)
+++ pretty.Rd	(working copy)
@@ -21,8 +21,8 @@
   \item{min.n}{nonnegative integer giving the \emph{minimal} number of
     intervals.  If \code{min.n == 0}, \code{pretty(.)} may return a
     single value.}
-  \item{shrink.sml}{positive numeric
-    by a which a default scale is shrunk in the case when
+  \item{shrink.sml}{positive numeric factor
+    by which a default scale is shrunk in the case when
     \code{range(x)} is very small (usually 0).}
   \item{high.u.bias}{non-negative numeric, typically \eqn{> 1}.
     The interval unit is determined as \{1,2,5,10\} times \code{b}, a