Fix apparent copy paste mistake
The note under the RUN header refers to the CMD instruction. It should refer to the RUN instruction instead.
@@ -238,9 +238,9 @@ commands using a base image that does not contain `/bin/sh`.
> **Note**:
> Unlike the *shell* form, the *exec* form does not invoke a command shell.
> This means that normal shell processing does not happen. For example,
-> `CMD [ "echo", "$HOME" ]` will not do variable substitution on `$HOME`.
+> `RUN [ "echo", "$HOME" ]` will not do variable substitution on `$HOME`.
> If you want shell processing then either use the *shell* form or execute
-> a shell directly, for example: `CMD [ "sh", "-c", "echo", "$HOME" ]`.
+> a shell directly, for example: `RUN [ "sh", "-c", "echo", "$HOME" ]`.
The cache for `RUN` instructions isn't invalidated automatically during
the next build. The cache for an instruction like
