Fix typo in the comments

tests/demo/stemdemo.cpp

@@ -21,7 +21,7 @@ int main()
     // the word to be stemmed
     std::wstring word(L"documentation");
 
-    /* Create an instance of a "english_stem" class. The template argument for the
+    /* Create an instance of an "english_stem" class. The template argument for the
        stemmers are the type of std::basic_string that you are trying to stem,
        by default std::wstring (Unicode strings).
        As long as the char type of your basic_string is wchar_t, then you can use