Fix markdown format and typo

- https://github.com/rails/rails/actions/runs/4319300757/jobs/7538324353
```
Offenses:

./rails/activesupport/CHANGELOG.md:34 CHANGELOG header must start with '*' and 3 spaces
*    Remove deprecated option to passing a format to `#to_s` in `Array`, `Range`, `Date`, `DateTime`, `Time`,
^^^^
13 changelogs inspected, 1 offense detected
Error: Process completed with exit code 1.
```
Follow-up e420c33

- https://github.com/rails/rails/actions/runs/4319300757/jobs/7538324760
```
./activestorage/test/models/variant_test.rb:200: ins't ==> isn't
```

Follow-up 4edaa41

activestorage/test/models/variant_test.rb

@@ -197,7 +197,7 @@ class ActiveStorage::VariantTest < ActiveSupport::TestCase
     end
   end
 
-  test "content_type not recognized by mini_mime ins't included as variable" do
+  test "content_type not recognized by mini_mime isn't included as variable" do
     blob = create_file_blob(filename: "racecar.jpg")
 
     # image/jpg is not recognized by mini_mime (image/jpeg is correct)

activesupport/CHANGELOG.md

@@ -31,7 +31,7 @@
 
     *Rafael Mendonça França*
 
-*    Remove deprecated option to passing a format to `#to_s` in `Array`, `Range`, `Date`, `DateTime`, `Time`,
+*   Remove deprecated option to passing a format to `#to_s` in `Array`, `Range`, `Date`, `DateTime`, `Time`,
     `BigDecimal`, `Float` and, `Integer`.
 
     *Rafael Mendonça França*