Fixed the attribute name for multiple licenses.

ext/io/nonblock/io-nonblock.gemspec

@@ -7,7 +7,7 @@ Gem::Specification.new do |spec|
   spec.summary       = %q{Enables non-blocking mode with IO class}
   spec.description   = %q{Enables non-blocking mode with IO class}
   spec.homepage      = "https://github.com/ruby/io-nonblock"
-  spec.license       = ["Ruby", "BSD-2-Clause"]
+  spec.licenses      = ["Ruby", "BSD-2-Clause"]
   spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
 
   spec.metadata["homepage_uri"] = spec.homepage

ext/io/wait/io-wait.gemspec

@@ -7,7 +7,7 @@ Gem::Specification.new do |spec|
   spec.summary       = %q{Waits until IO is readable or writable without blocking.}
   spec.description   = %q{Waits until IO is readable or writable without blocking.}
   spec.homepage      = "https://github.com/ruby/io-wait"
-  spec.license       = ["Ruby", "BSD-2-Clause"]
+  spec.licenses      = ["Ruby", "BSD-2-Clause"]
   spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
 
   spec.metadata["homepage_uri"] = spec.homepage