Fix typos (#26)

Found via `typos --hidden --format brief`

README.md

@@ -71,7 +71,7 @@ You can easily automate the process with ExDoc by adding the following to your `
 defmodule Azeroth.MixProject do
   use Mix.Project
 
-  # this is needed because the file are generated but if you run mix docs, Mix will check the existance of files first. so have to work around that
+  # this is needed because the file are generated but if you run mix docs, Mix will check the existence of files first. so have to work around that
   @graphql_files [
     "guides/graphql/enums.md",
     "guides/graphql/inputs.md",

lib/graphql_markdown/schema.ex

@@ -99,8 +99,8 @@ defmodule GraphqlMarkdown.Schema do
     Enum.filter(schema["types"], fn type -> !String.starts_with?(type["name"], "__") end)
   end
 
-  def schema_from_json(%{"data" => %{"__schema" => schema_defintion}}) do
-    {:ok, schema_defintion}
+  def schema_from_json(%{"data" => %{"__schema" => schema_definition}}) do
+    {:ok, schema_definition}
   end
 
   def schema_from_json(_) do

lib/mix/tasks/graphql.gen.markdown.ex

@@ -11,7 +11,7 @@ defmodule Mix.Tasks.GraphqlGenMarkdown do
 
   valid override args:
     -f, --schema            Path to the schema file
-    -o, --outpout-dir       Where to output the files. Default: [Current Directory]
+    -o, --output-dir        Where to output the files. Default: [Current Directory]
     -m, --multi-page        Generate each type in a separate file. Default: [single page format]
     -t, --title             Specify the page title for the generated file (Applies to single file only)
     --no-toc                Do not generate the table of content (Applies to single file only)