Correctly place the square of proof env using fenced divs environment in HTML

[cderv]

Following changes in #940 (comment)_, there is display issue in HTML with the square of the proof environment.

it happens because of the css rule currently used.

bookdown/inst/resources/gitbook/css/plugin-bookdown.css

Lines 93 to 96 in 57f1217

	div.proof:after {
	content: "\25a2";
	float: right;
	}

As using pandoc, everything is inserted between a <p> tag, this rule creates a floating square that is not placed where we want. Modifying the rule to add to the last <p> child should work for the new syntax, but there won't be a square printed in html for the old syntax... Not sure it is bug or loss...

[yihui]

I tested both syntaxes and the squares were shown correctly in the output:

```{proof}
The characteristic function of $X\sim\mathrm{Pois}(\lambda)$ is $\varphi _{X}(t)=e^{\lambda (e^{it}-1)}$. Let $P_n=\sum_{i=1}^nX_i$. We know from Theorem \@ref(thm:chf-sum) that

\begin{equation*}
\begin{split}
\varphi _{P_{n}}(t) & =\prod_{i=1}^n\varphi _{X_i}(t) \\
& =\prod_{i=1}^n e^{\lambda_i (e^{it}-1)} \\
& = e^{\sum_{i=1}^n \lambda_i (e^{it}-1)}
\end{split}
\end{equation*}

This is the characteristic function of a Poisson random variable with the parameter $\lambda=\sum_{i=1}^n \lambda_i$. From Lemma \@ref(lem:chf-pdf), we know the distribution of $P_n$ is $\mathrm{Pois}(\sum_{i=1}^n\lambda_i)$.
```

::: {.proof}
The characteristic function of $X\sim\mathrm{Pois}(\lambda)$ is $\varphi _{X}(t)=e^{\lambda (e^{it}-1)}$. Let $P_n=\sum_{i=1}^nX_i$. We know from Theorem \@ref(thm:chf-sum) that

\begin{equation*}
\begin{split}
\varphi _{P_{n}}(t) & =\prod_{i=1}^n\varphi _{X_i}(t) \\
& =\prod_{i=1}^n e^{\lambda_i (e^{it}-1)} \\
& = e^{\sum_{i=1}^n \lambda_i (e^{it}-1)}
\end{split}
\end{equation*}

This is the characteristic function of a Poisson random variable with the parameter $\lambda=\sum_{i=1}^n \lambda_i$. From Lemma \@ref(lem:chf-pdf), we know the distribution of $P_n$ is $\mathrm{Pois}(\sum_{i=1}^n\lambda_i)$.
:::

[cderv]

OK not sure what happens when I tried that. Seems ok now.

Thanks you !

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