Fix a typo

Author: seanchen1991

README.md

@@ -23,7 +23,7 @@
  passed in as an argument
 - [ ] Implement a Multiply instruction (run `mult8.ls8`)
 
-### Day 3; Stack
+### Day 3: Stack
 
 - [ ] Implement the System Stack and be able to run the `stack.ls8` program
 
@@ -36,4 +36,4 @@
 
 - [ ] Add the timer interrupt to the LS-8 emulator
 - [ ] Add the keyboard interrupt to the LS-8 emulator
-- [ ] Write an LS-8 assembly program to draw a curved histogram on the screen
+- [ ] Write an LS-8 assembly program to draw a curved histogram on the screen