Fix DL_WARN_documented_change URL

The doc_link argument is really a URL fragment within the
android-changes-for-ndk-developers.md document, not a filename at the
root of the bionic repository.

Test: manual
Bug: none
Change-Id: I1b542e47aca132ce43ba1d50d83db1bf3c7b10c6
diff --git a/linker/linker.h b/linker/linker.h
index 2da1404..9b6af3b 100644
--- a/linker/linker.h
+++ b/linker/linker.h
@@ -71,6 +71,10 @@
   DISALLOW_COPY_AND_ASSIGN(VersionTracker);
 };
 
+static constexpr const char* kBionicChangesUrl =
+    "https://android.googlesource.com/platform/bionic/+/master/"
+    "android-changes-for-ndk-developers.md";
+
 soinfo* get_libdl_info(const soinfo& linker_si);
 
 soinfo* find_containing_library(const void* p);