Fix documentation typos

Character entities are not supposed to be supported by gtk-doc¹ and
fix the spelling of ‘optional’

¹https://bugzilla.gnome.org/show_bug.cgi?id=758137

https://bugzilla.gnome.org/show_bug.cgi?id=758174

glib/gregex.c

@@ -64,7 +64,7 @@
  * Note that, unless you set the #G_REGEX_RAW flag, all the strings passed
  * to these functions must be encoded in UTF-8. The lengths and the positions
  * inside the strings are in bytes and not in characters, so, for instance,
- * "\xc3\xa0" (i.e. "à") is two bytes long but it is treated as a
+ * "\xc3\xa0" (i.e. "à") is two bytes long but it is treated as a
  * single character. If you set #G_REGEX_RAW the strings can be non-valid
  * UTF-8 strings and a byte is treated as a character, so "\xc3\xa0" is two
  * bytes and two characters long.