Remove unnecessary Vec<_> annotation from docs

This was brought up in IRC by a confused reader.

src/libcollections/vec.rs

@@ -185,7 +185,7 @@ impl<T> Vec<T> {
     /// # Examples
     ///
     /// ```
-    /// let mut vec: Vec<_> = Vec::with_capacity(10);
+    /// let mut vec = Vec::with_capacity(10);
     ///
     /// // The vector contains no items, even though it has capacity for more
     /// assert_eq!(vec.len(), 0);