fix spelling

Author: Hsn723

docs/extensions/toc.md

@@ -202,7 +202,7 @@ The following options are provided to configure the output:
 
     The callable must return a string appropriate for use in HTML `id` attributes.
 
-    An alternate version of the default callable supporting unicode strings is also provided as `slugify_unicode`.
+    An alternate version of the default callable supporting Unicode strings is also provided as `slugify_unicode`.
 
 * **`separator`**:
     Word separator. Character which replaces white space in id. Defaults to "`-`".