Fix SDK constraints 2.12.0-0 to 2.12.0 (#148)

diff --git a/pkgs/shelf_router/pubspec.yaml b/pkgs/shelf_router/pubspec.yaml
index d22b6a1..e22b6fb 100644
--- a/pkgs/shelf_router/pubspec.yaml
+++ b/pkgs/shelf_router/pubspec.yaml
@@ -15,4 +15,4 @@
   pedantic: ^1.10.0
   http: ^0.13.0
 environment:
-  sdk: '>=2.12.0-0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'