Extended Char Intro: Use getwc in example (Bug 25626)

In the "Extended Char Intro" the example incorrectly uses a function
called wgetc which doesn't exist.  The example is corrected to use
getwc, which is correct for the use in this case.

Reported-by: Toomas Rosin <toomas@rosin.ee>

manual/charset.texi

@@ -186,7 +186,7 @@ are used:
 @{
   wint_t c;
   @dots{}
-  while ((c = wgetc (fp)) != WEOF)
+  while ((c = getwc (fp)) != WEOF)
     @dots{}
 @}
 @end smallexample