Fix incorrect Beamer template names for `structureenv`

[Leo3418]

As per Section 12.2 of the Beamer User Guide for version 3.70, the Beamer template names for structureenv should be structure begin and structure end. However, some of the Beamer package's source files are not using these exact names.

An issue caused by this is that, in article mode, text in structureenv is not typeset in boldface as specified also by Section 12.2. A minimal working example is:

\documentclass{article}
\usepackage{beamerarticle}
\begin{document}
\begin{frame}
    \structure{This text should be typeset in boldface.}
\end{frame}
\end{document}

[samcarter]

@Leo3418 Thanks a lot for spotting this and your PR!