isogram: approaches: fix slug (#1588)

Commit e76c930 ("isogram: add approaches, 2022-11-08) added
approaches for the isogram exercise, but there was a typo in a `slug`
value.

In this situation, it is likely that an upcoming version of configlet
will produce an error like:

    $ configlet lint
    [...]
    A 'slug' value is 'bitfied', but there is no sibling directory with that name:
    ./exercises/practice/isogram/.approaches/config.json

Author: ee7

exercises/practice/isogram/.approaches/config.json

@@ -12,7 +12,7 @@
     },
     {
       "uuid": "ee6b5efb-7780-45f3-ae85-9a6ba4142e57",
-      "slug": "bitfied",
+      "slug": "bitfield",
       "title": "Bit field using a for loop",
       "blurb": "Use a bit field with a for loop to keep track of used letters.",
       "authors": ["bobahop"]