Fix spelling errors

content/04_pcb_milling.md

@@ -11,7 +11,7 @@ This week's topic is how to make printed circuit boards (PCBs). Our assignment i
 
 ### Milling
 
-We're using a Roland Modela MDX-20 milling machine, controlled with [mods](http://mods.cba.mit.edu/). The mods interface takes black and white png images as input, calculates toolpaths, and sends the resulting G-code to the milling machine.
+We're using a Roland Modela MDX-20 milling machine, controlled with [mods](http://mods.cba.mit.edu/). The mods interface takes black and white PNG images as input, calculates toolpaths, and sends the resulting G-code to the milling machine.
 
 ![](/img/04_mods.jpg#c)
 
@@ -30,7 +30,7 @@ After the test pattern, I milled a programmer.
 
 ### Stuffing
 
-I had never tried to solder surface mount components before, so I wasn't sure how it would go. My first attempt was a dissapointment: crystallized joints across the board (ha).
+I had never tried to solder surface mount components before, so I wasn't sure how it would go. My first attempt was a disappointment: crystallized joints across the board (ha).
 
 ![](/img/04_lead_free.jpg#c)