Fix typo.

git-svn-id: https://svn.r-project.org/R/trunk@87074 00db46b3-68df-0310-9c12-caf00c1e9a41

doc/manual/R-exts.texi

@@ -14369,8 +14369,8 @@ Weak references are used to allow the programmer to maintain information
 on entities without preventing the garbage collection of the entities
 once they become unreachable.
 
-A weak reference contains a key and a value. The value is reachable is
-if it either reachable directly or @emph{via} weak references with reachable
+A weak reference contains a key and a value. The value is reachable
+if it is either reachable directly or @emph{via} weak references with reachable
 keys. Once a value is determined to be unreachable during garbage
 collection, the key and value are set to @code{R_NilValue} and the
 finalizer will be run later in the garbage collection.