Fix README.md: make examples compile and make more consistent

[Pivok7]

I decided to check and test the README.md more in depth and found several issues so i fixed them.

• In my previous commit i missed that vulkan integration example has the problem with non debug builds (mentioned in here Wont compile with anything but debug #35) and added .target and .optimize fields to fix that.

• Vulkan headers line was missing a second parameter

                                                      vvv here
const vulkan_headers = b.dependency("vulkan_headers", .{}); 

• create window example had a missing fifth parameter

Cosmetic changes

• I changed the Vulkan dependency name from "vulkan_zig" to "vulkan" as that's how it appears in build.zig.zon by default after fetching.

• I changed the first build.zig example to be more self contained and copy-paste friendly

• I made the Vulkan integration example more concise and more similar to the original build.zig mentioned above so that it can be updated with minimal effort.

[hazeycode]

Thanks!