Consistent indentation

This was done by executing this command:

```
find src -type f -name '*.rst' -exec sed -Ei 's/^  :/    :/g' {} \;
```

src/building-build-configs/sections/templates.rst

@@ -6,7 +6,7 @@ Dockerfile.twig
 ===============
 
 .. code-block:: twig
-  :linenos:
+    :linenos:
 
     FROM php:{{ php.version }} AS base