overview: slightly modernize

Author: andrewrk

content/en-US/learn/overview.smd

@@ -107,7 +107,7 @@ Unadorned Zig pointers cannot be null:
 
 []($code.language('=html').buildAsset('features/6-null-to-ptr.zig'))
 
-However any type can be made into an [optional type](https://ziglang.org/documentation/master/#Optionals) by prefixing it with ?:
+However any type can be made into an [optional type](https://ziglang.org/documentation/master/#Optionals) by prefixing it with `?`:
 
 []($code.language('=html').buildAsset('features/7-optional-syntax.zig'))
 
@@ -179,7 +179,7 @@ In addition, the standard library has the ability to capture a stack trace at an
 
 []($code.language('=html').buildAsset('features/17-stack-traces.zig'))
 
-You can see this technique being used in the ongoing [GeneralPurposeDebugAllocator project](https://github.com/andrewrk/zig-general-purpose-allocator/#current-status).
+The standard library's DebugAllocator uses this technique to report leaks and double frees.
 
 ## Generic data structures and functions