Update pubspec.yaml (#46)

diff --git a/pubspec.yaml b/pubspec.yaml
index d34c830..b16fb22 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,7 +4,7 @@
 homepage: https://github.com/google/pub_cache
 
 environment:
-  sdk: '>=2.12.0-0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   path: ^1.8.0