Fix typos

It is newly detected by misspell.

```console
Run codespell --ignore-words=codespell.txt || exit 1
./README.adoc:4310: falsy ==> falsely, false
./README.adoc:4313: falsy ==> falsely, false
Error: Process completed with exit code 1.
```

https://github.com/rubocop/ruby-style-guide/runs/7968170911

Author: koic

README.adoc

@@ -4307,10 +4307,10 @@ Introduce default values for hash keys via `Hash#fetch` as opposed to using cust
 ----
 batman = { name: 'Bruce Wayne', is_evil: false }
 
-# bad - if we just use || operator with falsy value we won't get the expected result
+# bad - if we just use || operator with falsey value we won't get the expected result
 batman[:is_evil] || true # => true
 
-# good - fetch works correctly with falsy values
+# good - fetch works correctly with falsey values
 batman.fetch(:is_evil, true) # => false
 ----