fix typo (#129)

Author: PeterWrighten

src/safe-guides/coding_practice/fn-design/G.FUD.06.md

@@ -13,7 +13,7 @@
 fn three_vowels(word: &String) -> bool {
     let mut vowel_count = 0;
     for c in word.chars() {
-        matchc {
+        match c {
             'a' | 'e' | 'i' | 'o' | 'u' => {
                 vowel_count += 1;
                 if vowel_count >= 3 {