Fix typo semverg -> semver

Originally from pull request #41 by @seungwonpark.

Signed-off-by: JP-Ellis <josh@jpellis.me>

tikz-feynman.tex

@@ -452,7 +452,7 @@ \section{Introduction}
 \href{https://github.com/JP-Ellis/tikz-feynman}{Github}\footnote{\url{https://github.com/JP-Ellis/tikz-feynman}}.
 
 \tikzfeynmanname{}'s versioning will approximately follow
-\href{http://semverg.org}{semantic versioning}.  This means that changes in the
+\href{http://semver.org}{semantic versioning}.  This means that changes in the
 third number (|1.0.0| to |1.0.1|) will consist of bug fixes and very minor
 changes but they should not change the output otherwise\footnote{That is, with
   the exception of the bug that they are fixing.}.  Changes in the second number