renamed 'doc' to 'static'

- signed-off-by: trimstray <trimstray@gmail.com>

README.md

@@ -1,5 +1,5 @@
 <p align="center">
-    <img src="https://github.com/trimstray/the-book-of-secret-knowledge/blob/master/doc/img/the-book-of-secret-knowledge-preview.png"
+    <img src="https://github.com/trimstray/the-book-of-secret-knowledge/blob/master/static/img/the-book-of-secret-knowledge-preview.png"
         alt="Master">
 </p>