Fix style in true/false/nil singleton_method specs

* And add parens around receiver to avoid parse errors on some Rubies.

Author: eregon

spec/ruby/core/false/singleton_method_spec.rb

@@ -3,10 +3,10 @@
 describe "FalseClass#singleton_method" do
   ruby_version_is '3.3' do
     it "raises regardless of whether FalseClass defines the method" do
-      proc{false.singleton_method(:foo)}.should raise_error(NameError)
+      -> { false.singleton_method(:foo) }.should raise_error(NameError)
       begin
-        def false.foo; end
-        proc{false.singleton_method(:foo)}.should raise_error(NameError)
+        def (false).foo; end
+        -> { false.singleton_method(:foo) }.should raise_error(NameError)
       ensure
         FalseClass.send(:remove_method, :foo)
       end

spec/ruby/core/nil/singleton_method_spec.rb

@@ -3,10 +3,10 @@
 describe "NilClass#singleton_method" do
   ruby_version_is '3.3' do
     it "raises regardless of whether NilClass defines the method" do
-      proc{nil.singleton_method(:foo)}.should raise_error(NameError)
+      -> { nil.singleton_method(:foo) }.should raise_error(NameError)
       begin
-        def nil.foo; end
-        proc{nil.singleton_method(:foo)}.should raise_error(NameError)
+        def (nil).foo; end
+        -> { nil.singleton_method(:foo) }.should raise_error(NameError)
       ensure
         NilClass.send(:remove_method, :foo)
       end

spec/ruby/core/true/singleton_method_spec.rb

@@ -3,10 +3,10 @@
 describe "TrueClass#singleton_method" do
   ruby_version_is '3.3' do
     it "raises regardless of whether TrueClass defines the method" do
-      proc{true.singleton_method(:foo)}.should raise_error(NameError)
+      -> { true.singleton_method(:foo) }.should raise_error(NameError)
       begin
-        def true.foo; end
-        proc{true.singleton_method(:foo)}.should raise_error(NameError)
+        def (true).foo; end
+        -> { true.singleton_method(:foo) }.should raise_error(NameError)
       ensure
         TrueClass.send(:remove_method, :foo)
       end