Replace master with main in URLs & docs
diff --git a/docs/implementing_proto3_presence.md b/docs/implementing_proto3_presence.md
index ae8b369..7c3e213 100644
--- a/docs/implementing_proto3_presence.md
+++ b/docs/implementing_proto3_presence.md
@@ -30,7 +30,7 @@
inside Google and [from open-source
users](https://github.com/protocolbuffers/protobuf/issues/1606). The [proto3
wrapper
-types](https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/wrappers.proto)
+types](https://github.com/protocolbuffers/protobuf/blob/main/src/google/protobuf/wrappers.proto)
were previously the only supported presence mechanism for proto3. Users have
pointed to both efficiency and usability issues with the wrapper types.
