Add a missing trailing "." to an error message

Author: nex3

spec/libsass-closed-issues/issue_1452.hrx

@@ -9,7 +9,7 @@ Error: () isn't a valid CSS value.
   Use --trace for backtrace.
 
 <===> error-dart-sass
-Error: () isn't a valid CSS value
+Error: () isn't a valid CSS value.
   ,
 2 |   foo: foo(());
   |            ^^