change output file extension (config.txt -> config.json)

This is causing some build errors because the config.json file isn't
associated with a rule (because the rule looks for .txt).

Test: ran locally, checked output directory
Bug: 142551000
Change-Id: Idcdb226fa2d23055a8850b2a0545202921edc3b1
diff --git a/cc/fuzz.go b/cc/fuzz.go
index 7fee84e..577fa70 100644
--- a/cc/fuzz.go
+++ b/cc/fuzz.go
@@ -149,7 +149,7 @@
 	}
 
 	if fuzz.Properties.Fuzz_config != nil {
-		configPath := android.PathForModuleOut(ctx, "config").Join(ctx, "config.txt")
+		configPath := android.PathForModuleOut(ctx, "config").Join(ctx, "config.json")
 		ctx.Build(pctx, android.BuildParams{
 			Rule:        android.WriteFile,
 			Description: "fuzzer infrastructure configuration",