Update README.md

Example in README couldn't compile. Compilation error : 

 `PullOptions` is the name of a struct or struct variant, but this expression uses it like a function name [E0423]
&PullOptions.builder().image("redis:2.8.18").build()

1 files changed, 1 insertions, 1 deletions

diff --git a/README.md b/README.md
index 8edfb99..2ea8196 100644
--- a/README.md
+++ b/README.md
@@ -78,7 +78,7 @@ for i in image.search("rust").unwrap() {
 ```rust
 use shiplift::PullOptions;
 let output = images.pull(
-  &PullOptions.builder().image("redis:2.8.18").build()
+  &PullOptions::builder().image("redis:2.8.18").build()
 ).unwrap();
 for o in output {
   println!("{:?}", o);