Fix header filename in Overview (#1459)

Summary: Pull Request resolved: https://github.com/facebook/folly/pull/1459

Reviewed By: yfeldblum

Differential Revision: D25730366

Pulled By: Orvid

fbshipit-source-id: d0fe94b1db6d81c0dabaa65de1483ea319669776

folly/docs/Overview.md

@@ -109,7 +109,7 @@ processing sequences of values efficiently in C++ akin to C#'s LINQ.
 encoding](http://www.ir.uwaterloo.ca/book/addenda-06-index-compression.html)
 for 32-bit values.
 
-#### `IpAddress.h`
+#### `IPAddress.h`
 
 A collection of utilities to deal with IPAddresses, including ipv4 and ipv6.