Links in README.md, take two.

gitiles seems to mangle the links when viewed from
https://boringssl.googlesource.com/boringssl/+/master/README.md, but not
https://boringssl.googlesource.com/boringssl/.

This document suggests that it handles absolute and relative links to files
specially. I guess it didn't interpret 'README.md' as either and failed to adjust
it accordingly. Let's see if this works instead.

https://gerrit.googlesource.com/gitiles/+/master/Documentation/markdown.md#Links

Change-Id: Ic181919140f968c88c69ea8f3c21c70f63e4b467
Reviewed-on: https://boringssl-review.googlesource.com/6230
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/README.md b/README.md
index 2b3e9f3..142fc0a 100644
--- a/README.md
+++ b/README.md
@@ -23,7 +23,7 @@
 
 There are other files in this directory which might be helpful:
 
-  * [PORTING.md](PORTING.md): how to port OpenSSL-using code to BoringSSL.
-  * [BUILDING.md](BUILDING.md): how to build BoringSSL
-  * [STYLE.md](STYLE.md): rules and guidelines for coding style.
+  * [PORTING.md](/PORTING.md): how to port OpenSSL-using code to BoringSSL.
+  * [BUILDING.md](/BUILDING.md): how to build BoringSSL
+  * [STYLE.md](/STYLE.md): rules and guidelines for coding style.
   * include/openssl: public headers with API documentation in comments. Also [available online](https://commondatastorage.googleapis.com/chromium-boringssl-docs/headers.html).