Fix Skylark documentation links

I thought Jekyll would translate these, but apparently not?

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MOS_MIGRATED_REVID=112977415

site/docs/skylark/index.md

diff --git a/site/docs/skylark/index.md b/site/docs/skylark/index.md
index 92f53ad..bc20bea 100644
--- a/site/docs/skylark/index.md
+++ b/site/docs/skylark/index.md
@@ -5,8 +5,8 @@
 # Custom rules
 
 Skylark is the code name of the extension mechanism for Bazel. It lets you write
-[custom build rules](rules.md) as well as compose existing ones into
-[macros](macros.md).
+[custom build rules](rules.html) as well as compose existing ones into
+[macros](macros.html).
 
 
 ## Getting started