Fix last .markdown file to be a .md file

This was also in one of my entries so I know it's fine to change.

2018/ferguson/rpm.md

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+What? Install an IOCCC entry? I realise it seems odd so why did I make a rpm
+spec file and why did I hint at building RPMs for others? I can't say precisely;
+**certainly I'm under no impression that anyone else will want to use it** *but
+the entry was fun enough for me that I actually will* (a number of my programs
+over the years are programs I actually install and I already have done so with
+this entry). I already have my own RPM repository and initially after having won
+I set it up so that I could host the weasel rpm (binary and source) there. At
+the same time though I feel the entry belongs on the IOCCC website; that is to
+say the only place the raw files should be downloadable from is the IOCCC
+website itself. Simon Cooper (the judge who deals with setting up the entries on
+the IOCCC website) asked me if there is any real attachment in having the RPM
+built as part of the entry and the answer is 'no'. He did suggest too that if
+I were to have the RPM hosted on the IOCCC website not only would it place a
+burden on me to keep it up to date but it would place a burden on the judges -
+in other words he himself. Even if it wasn't a burden to me I agree on the other
+part and so I have instead come to the following ideas:
+
+
+*   I will not host the actual source code or scripts on my website; they will
+remain on the IOCCC website. This will most likely never change; instead I would
+link to the entry directory so that they remain where they belong: the IOCCC
+website itself. I have set up an [errata page][] however should there be anything
+requiring a fix.
+
+*   I am keeping the weasel.spec file in the entry as it was part of my
+submission but I do not explain here how to set up the build environment or how
+to build a RPM: if you know how to use **`rpmbuild`** then you should be able to
+work it out.
+
+*   Because I strongly suspect I am the only one who has any desire for actually
+installing an IOCCC entry I will not worry about setting up a repository; if I
+do set up a repository in the future I will include information on it somewhere
+at [https://ioccc.xexyl.net/2018/weasel][].
+
+[errata page]: https://ioccc.xexyl.net/2018/weasel/errata
+[https://ioccc.xexyl.net/2018/weasel]: https://ioccc.xexyl.net/2018/weasel