Fix apparent copy paste mistake

The note under the RUN header refers to the CMD instruction. It should refer to the RUN instruction instead.

Signed-off-by: Huu Nguyen <whoshuu@gmail.com>

docs/sources/reference/builder.md

@@ -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