Use PNG instead of SVG for address image

The SVG renders incorrectly in Chrome. See https://github.com/phil-opp/blog_os/pull/517#discussion_r247489190 .

blog/content/second-edition/posts/09-paging-introduction/index.md

@@ -156,7 +156,7 @@ The physical address of the currently active level 4 page table, which is the ro
 
 The above page table hierarchy maps two pages (in blue). From the page table indices we can deduce that the virtual addresses of these two pages are `0x803FE7F000` and `0x803FE00000`. Let's see what happens when the program tries to read from address `0x803FE7F5CE`. First, we convert the address to binary and determine the page table indices and the page offset for the address:
 
-![The sign extension bits are all 0, the level 4 index is 1, the level 3 index is 0, the level 2 index is 511, the level 1 index is 127, and the page offset is 0x5ce](x86_64-page-table-translation-addresses.svg)
+![The sign extension bits are all 0, the level 4 index is 1, the level 3 index is 0, the level 2 index is 511, the level 1 index is 127, and the page offset is 0x5ce](x86_64-page-table-translation-addresses.png)
 
 With these indices, we can now walk the page table hierarchy to determine the mapped frame for the address: