Say "last" instead of "rightmost" in the documentation for std::str::rfind.

In the documentation comment for `std::str::rfind`, say "last" instead
of "rightmost" to describe the match that `rfind` finds. This follows the
spirit of rust-lang#30459, for which `trim_left` and `trim_right` were replaced by
`trim_start` and `trim_end` to be more clear about how they work on
text which is displayed right-to-left.

Author: sunfishcode

library/core/src/str/mod.rs

@@ -1158,7 +1158,7 @@ impl str {
         pat.into_searcher(self).next_match().map(|(i, _)| i)
     }
 
-    /// Returns the byte index for the first character of the rightmost match of the pattern in
+    /// Returns the byte index for the first character of the last match of the pattern in
     /// this string slice.
     ///
     /// Returns [`None`] if the pattern doesn't match.