docs: fix unwanted highlighting.

Plain text example lines were being formatted as python. Now formatted
as plain text.

Author: rouilj

doc/user_guide.txt

@@ -628,6 +628,8 @@ items. At the end of the subject line, propname=value pairs can be
 specified in square brackets, using the same conventions as for the
 roundup set shell command.
 
+.. highlight:: text
+
 For example,
 
 - setting the priority of an issue::
@@ -646,6 +648,8 @@ For example,
 
    Subject: Re: [ issue 2 ] we're out of widgets [nosy=-richard;priority=bug]
 
+.. highlight:: python3
+
 In all cases, the message relates to issue 2. The ``Re:`` prefix is
 stripped off.