Allow running build-man.sh from any directory

Before:

    $ src/doc/build-man.sh
    error: manifest path `../../crates/mdman/Cargo.toml` does not exist

After: works.

Author: dtolnay

src/doc/build-man.sh

@@ -12,6 +12,8 @@
 
 set -e
 
+cd "$(dirname "${BASH_SOURCE[0]}")"
+
 OPTIONS="--url https://doc.rust-lang.org/cargo/commands/ \
     --man rustc:1=https://doc.rust-lang.org/rustc/index.html \
     --man rustdoc:1=https://doc.rust-lang.org/rustdoc/index.html"