manual: Fix indentation in code blocks

mu4e/mu4e.texi

@@ -2891,7 +2891,7 @@ Defining a new custom action comes down to writing an elisp-function to do the
 work. Functions that operate on messages receive a @var{msg} parameter, which
 corresponds to the message at point. Something like:
 @lisp
-  (defun my-action-func (msg)
+(defun my-action-func (msg)
   "Describe my message function."
   ;; do stuff
   )
@@ -2903,7 +2903,7 @@ corresponds to the message at point, and an @var{attachment-num}, which is the
 number of the attachment as seen in the message view. An attachment function
 looks like:
 @lisp
-  (defun my-attachment-action-func (msg attachment-num)
+(defun my-attachment-action-func (msg attachment-num)
   "Describe my attachment function."
   ;; do stuff
   )