Update embedding.md to correct typo (#4188)

there is a repeating word.


**What type of PR is this?**

Documentation

**What does this PR do? Why is it needed?**

might help with clarity

**Which issues(s) does this PR fix?**

Fixes #

**Other notes for review**

Co-authored-by: Zhongpeng Lin <zplin@uber.com>

docs/go/core/embedding.md

@@ -61,7 +61,7 @@ go_test(
 )
 ```
 
-Embedding may also be used to add extra sources sources to a
+Embedding may also be used to add extra sources to a
 `go_proto_library`.
 
 ``` bzl