Fix typo in docs and add small grammar improvement to changelog

Author: ethancrawford

CHANGELOG.md

@@ -82,7 +82,7 @@ Have fun and happy live coding!
   modulation. This modifies the rate to match the specified number of
   MIDI notes relative from the current pitch. For example, a `rpitch:`
   of 12 would double the rate.
-* The units of the FX `:echo`'s `decay:` opt is now beats and is scaled
+* The units of the FX `:echo`'s `decay:` opt are now beats and are scaled
   with the BPM.
 
 

app/server/sonicpi/lib/sonicpi/mods/sound.rb

@@ -863,7 +863,7 @@ def set_mixer_control!(opts)
           summary:       "Control master mixer",
           doc:           "The master mixer is the final mixer that all sound passes through. This fn gives you control over the master mixer allowing you to manipulate all the sound playing through Sonic Pi at once. For example, you can sweep a lpf or hpf over the entire sound.",
           args:          [],
-          opts:          {pre_amp:        "Controls the amplitude of the signal prior to th e FX stage of the mixer (prior to lpf/hpf stages). Has slide opts. Default 1.",
+          opts:          {pre_amp:        "Controls the amplitude of the signal prior to the FX stage of the mixer (prior to lpf/hpf stages). Has slide opts. Default 1.",
                           amp:            "Controls the amplitude of the signal after the FX tage. Has slide opts. Default 1.",
                           hpf:            "Global hpf FX. Has slide opts. Default 0.",
                           lpf:            "Global lpf FX. Has slide opts. Default 135.5.",