Fix README.

Author: yuuu

README.md

@@ -1,6 +1,6 @@
 # mruby-esp32-mcpwm
 
-PWM library for mruby-esp32.
+MCPWM library for mruby-esp32.
 
 ## Installation
 
@@ -16,14 +16,14 @@ Add the line below to your `build_config.rb`:
 
 ```ruby
 mcpwm = MCPWM.new(1)  # GPIO pin = 1
-mcpwm.frequency(50) # set frequency(Hz) of PWM
-mcpwm.duty(10)      # set duty(%) of PWM
+mcpwm.frequency(50)   # set frequency(Hz) of MCPWM
+mcpwm.duty(10)        # set duty(%) of MCPWM
 # => drive with frequency=50Hz(Period=20_000us), duby=10%(Pulse Width=2000us)
 
 mcpwm = MCPWM.new(1, freq: 100, duty: 40)
 # => drive with frequency=100Hz(Period=10_000us), duby=40%(Pulse Width=4000us)
 
-mcpwm = PWM.new(1)
+mcpwm = MCPWM.new(1)
 mcpwm.period_us(25_000)
 mcpwm.pulse_width_us(5_000)
 # => drive with frequency=40Hz(Period=25_000us), duby=20%(Pulse Width=5000us)