fix the same typo in doctest

Author: lcolaholicl

src/libcore/char/methods.rs

@@ -341,12 +341,12 @@ impl char {
     ///
     /// // both can be represented as three bytes
     /// assert_eq!(3, eastern.len_utf8());
-    /// assert_eq!(3, capitol.len_utf8());
+    /// assert_eq!(3, capital.len_utf8());
     ///
     /// // as a &str, these two are encoded in UTF-8
     /// let tokyo = "東京";
     ///
-    /// let len = eastern.len_utf8() + capitol.len_utf8();
+    /// let len = eastern.len_utf8() + capital.len_utf8();
     ///
     /// // we can see that they take six bytes total...
     /// assert_eq!(6, tokyo.len());