docs(readme): use absolute URLs for logo images

Use https://raw.githubusercontent.com/<org>/<repo>/main/... for logo
references so they render on crates.io. Relative paths render correctly
on GitHub but are dropped by the crates.io README sanitizer, leaving a
broken image at the top of every crate page.

Author: nh13

README.md

@@ -11,9 +11,9 @@ A pooled writer and compressor.
 <p>
 <a href="https://fulcrumgenomics.com">
 <picture>
-  <source media="(prefers-color-scheme: dark)" srcset=".github/logos/fulcrumgenomics-dark.svg">
-  <source media="(prefers-color-scheme: light)" srcset=".github/logos/fulcrumgenomics-light.svg">
-  <img alt="Fulcrum Genomics" src=".github/logos/fulcrumgenomics-light.svg" height="100">
+  <source media="(prefers-color-scheme: dark)" srcset="https://raw.githubusercontent.com/fulcrumgenomics/pooled-writer/main/.github/logos/fulcrumgenomics-dark.svg">
+  <source media="(prefers-color-scheme: light)" srcset="https://raw.githubusercontent.com/fulcrumgenomics/pooled-writer/main/.github/logos/fulcrumgenomics-light.svg">
+  <img alt="Fulcrum Genomics" src="https://raw.githubusercontent.com/fulcrumgenomics/pooled-writer/main/.github/logos/fulcrumgenomics-light.svg" height="100">
 </picture>
 </a>
 </p>