note_buffer: Don't use the length variable

It's not accurate. It's probably expressed in bytes.
It's equal to 1 only for normal characters, but 2 for special ones
such as é, è...etc.

Fixes linuxmint/mint21.3-beta#28

usr/lib/sticky/note_buffer.py

@@ -649,7 +649,7 @@ def do_insert_text(self, location, text, length):
             next_char.forward_char()
 
             # check tag toggles
-            if length == 1:
+            if len(text) == 1:
                 font_scale_changed = False
                 for tag_name in self.tag_toggles:
                     # if the tag is a font scale, remove all other font scales