Fix mrbgem.rake

Author: yuuu

mrbgem.rake

@@ -1,7 +1,7 @@
-MRuby::Gem::Specification.new('mruby-esp32-pwm') do |spec|
+MRuby::Gem::Specification.new('mruby-esp32-mcpwm') do |spec|
   spec.license = 'MIT'
   spec.author  = 'yuuu'
-  spec.summary = 'PWM class on ESP32'
+  spec.summary = 'MCPWM class on ESP32'
 
   spec.add_dependency('mruby-rational', :core => 'mruby-rational')
 end