Swap "libpng12-dev" for "libpng-dev" metapackage in PHP example

Closes https://github.com/docker-library/php/issues/485

php/content.md

@@ -95,7 +95,7 @@ RUN apt-get update && apt-get install -y \
 		libfreetype6-dev \
 		libjpeg62-turbo-dev \
 		libmcrypt-dev \
-		libpng12-dev \
+		libpng-dev \
 	&& docker-php-ext-install -j$(nproc) iconv mcrypt \
 	&& docker-php-ext-configure gd --with-freetype-dir=/usr/include/ --with-jpeg-dir=/usr/include/ \
 	&& docker-php-ext-install -j$(nproc) gd