Add coverage.txt to gitignore

Signed-off-by: Robert Lin <robertlin1@gmail.com>

.gitignore

@@ -3,6 +3,7 @@ cmd/hercules/plugin_template_source.go
 contrib/_plugin_example/churn_analysis.pb.go
 pb/pb.pb.go
 pb/pb_pb2.py
+coverage.txt
 
 **/.DS_Store
 .idea

coverage.txt

@@ -1 +0,0 @@
-mode: atomic