book: Fix building with mdBook v0.5

Author: AfoHT

book/en/book.toml

@@ -6,7 +6,6 @@ authors = [
   "Jorge Aparicio <jorge@japaric.io>",
   "Per Lindgren <per.lindgren@ltu.se>",
 ]
-multilingual = false
 src = "src"
 title = "Real-Time Interrupt-driven Concurrency"
 
@@ -18,5 +17,5 @@ command = "mdbook-mermaid"
 
 [output.html]
 git-repository-url = "https://github.com/rtic-rs/rtic"
-git-repository-icon = "fa-github"
+git-repository-icon = "fab-github"
 additional-js = ["mermaid.min.js", "mermaid-init.js"]