Update pubspec.yaml (dart-lang/pub_semver#64)

diff --git a/pkgs/pub_semver/pubspec.yaml b/pkgs/pub_semver/pubspec.yaml
index 99392f5..aa7ab52 100644
--- a/pkgs/pub_semver/pubspec.yaml
+++ b/pkgs/pub_semver/pubspec.yaml
@@ -6,7 +6,7 @@
 repository: https://github.com/dart-lang/pub_semver
 
 environment:
- sdk: '>=2.12.0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   collection: ^1.15.0