Fix broken links

Merged: https://github.com/ruby/ruby/pull/7979

README.ja.md

@@ -152,7 +152,7 @@ UNIXであれば `configure` がほとんどの差異を吸収してくれるは
 
 ## 配布条件
 
-[COPYING.ja](COPYING.ja) ファイルを参照してください．
+[COPYING.ja](https://docs.ruby-lang.org/en/master/COPYING_ja.html) ファイルを参照してください．
 
 ## フィードバック
 

README.md

@@ -53,7 +53,7 @@ if you are a committer.
 
 ## How to build
 
-see [Building Ruby](doc/contributing/building_ruby.md)
+See [Building Ruby](https://docs.ruby-lang.org/en/master/contributing/building_ruby_md.html)
 
 ## Ruby home page
 

doc/contributing/building_ruby.md

@@ -25,7 +25,8 @@
     * libffi (to build fiddle)
     * gmp (if you with to accelerate Bignum operations)
     * libexecinfo (FreeBSD)
-    * rustc - 1.58.0 or later (if you wish to build [YJIT](/doc/yjit/yjit.md))
+    * rustc - 1.58.0 or later, if you wish to build
+      [YJIT](https://docs.ruby-lang.org/en/master/RubyVM/YJIT.html).
 
     If you installed the libraries needed for extensions (openssl, readline, libyaml, zlib) into other than the OS default place,
     typically using Homebrew on macOS, add `--with-EXTLIB-dir` options to `CONFIGURE_ARGS` environment variable.