change 'log' -> 'logrus'

Change 'log' -> 'logrus' to fix syntax error

README.md

diff --git a/README.md b/README.md
index 6f04c8a..5c6fc38 100644
--- a/README.md
+++ b/README.md
@@ -58,7 +58,7 @@
 )
 
 func main() {
-  log.WithFields(log.Fields{
+  log.WithFields(logrus.Fields{
     "animal": "walrus",
   }).Info("A walrus appears")
 }