Rename directory to match crate name.

mdbook-exerciser/README.md

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+# exerciser
+
+This is an mdBook renderer to generate templates for exercises from the Markdown source. Given a
+Markdown file `example.md` with one or more sections like:
+
+````markdown
+<!-- File src/main.rs -->
+
+```rust,compile_fail
+{{#include example/src/main.rs:main}}
+
+fn some_more_code() {
+    // TODO: Write some Rust code here.
+}
+```
+````
+
+It will create a file `book/exerciser/exercise-templates/example/src/main.rs` with the appropriate
+contents.