Update mdbook to 0.4.28 (google#480)

* Update mdbook to 0.4.28

This version contains rust-lang/mdBook#1986 which will allow us to test the code in each translation.

* Fix formatting in Korean translation

The extra code block made `mdbook test` fail.

Author: mgeisler

.github/workflows/install-mdbook/action.yml

@@ -8,7 +8,7 @@ runs:
     # The --locked flag is important for reproducible builds. It also
     # avoids breakage due to skews between mdbook and mdbook-svgbob.
     - name: Install mdbook
-      run: cargo install mdbook --locked --version 0.4.25
+      run: cargo install mdbook --locked --version 0.4.28
       shell: bash
 
     - name: Install mdbook-svgbob

po/ko.po

@@ -8273,12 +8273,10 @@ msgid ""
 "    Self: Sized`"
 msgstr ""
 "`Iterator`는 아래와 같이 구현됩니다.\n"
-"```\n"
-"fn collect<B>(self) -> B\n"
+"`fn collect<B>(self) -> B\n"
 "where\n"
 "    B: FromIterator<Self::Item>,\n"
-"    Self: Sized\n"
-"```"
+"    Self: Sized`"
 
 #: src/traits/from-iterator.md:23
 msgid ""