'a' -> 'an'

This one could go either way, I guess. If most folk read __init__ like I
do, simply as the word "init" then this is needed; likewise if they say the
underscore, on the other hand if folk tend to go with dunder-init...)

I may recant this depending on the result of this:

  https://twitter.com/davepdotorg/status/1581331978049052672

docs/guide/reactivity.md

@@ -35,7 +35,7 @@ The `reactive` constructor accepts a default value as the first positional argum
 
     Textual uses Python's _descriptor protocol_ to create reactive attributes, which is the same protocol used by the builtin `property` decorator.
 
-You can get and set these attributes in the same way as if you had assigned them in a `__init__` method. For instance `self.name = "Jessica"`, `self.count += 1`, or `print(self.is_cool)`.
+You can get and set these attributes in the same way as if you had assigned them in an `__init__` method. For instance `self.name = "Jessica"`, `self.count += 1`, or `print(self.is_cool)`.
 
 ### Dynamic defaults