fix very minor punctuation typo

src/overview.md

@@ -177,7 +177,7 @@ satisfy/optimize for. For example,
       of space on the user's system...
 - Compiler memory usage: while compiling a program, we don't want to use more
   memory than we need.
-- Program speed: how fast is your compiled program. More/better compile-time
+- Program speed: how fast is your compiled program? More/better compile-time
   analyses often means the compiler can do better optimizations.
 - Program size: how large is the compiled binary? Similar to the previous
   point.