doc: fix adoc deflayermap underscore character display (#885)

Before:

![image](https://github.com/jtroo/kanata/assets/650565/9488169f-ad63-45e4-8aa5-4f47290f46b2)

After:

![image](https://github.com/jtroo/kanata/assets/650565/5b619353-cc03-4a61-905c-6ba6ef6cfda5)

Solution found at asciidoctor/asciidoctor#3528

docs/config.adoc

@@ -182,13 +182,15 @@ The input key takes the same role as `defsrc` keys.
 The output action takes the role that items in the normal `deflayer` have.
 
 Instead of specifying an input key,
-you can use either `_`, `__`, or `___` to map all
+you can use either `_`, `__`, or `___` to map all
 the keys that are not explicitly mapped in the layer
 (`caps` in the example above). 
 
-* `_` maps only keys that are in defsrc.
-* `__` excludes mapping keys that are in defsrc.
-* `___` maps all keys that are not explicitly mapped in the layer.
+`_` maps only keys that are in defsrc.
+
+`__` excludes mapping keys that are in defsrc.
+
+`___` maps all keys that are not explicitly mapped in the layer.
 
 
 The map string can be any of the following strings, to your liking: