README.md: Make docs URL a link

Dedent URL so it renders as a link and not as code (fixed width font and not clickable).

README.md

@@ -74,4 +74,4 @@ Usage
 The complete documentation for Protocol Buffers is available via the
 web at:
 
-    https://developers.google.com/protocol-buffers/
+https://developers.google.com/protocol-buffers/