Fix build note for MinGW

Closes sfackler#689

Author: sfackler

openssl-sys/build.rs

@@ -176,7 +176,7 @@ installing `pkg-config`.
 It looks like you're compiling for MinGW but you may not have either OpenSSL or
 pkg-config installed. You can install these two dependencies with:
 
-    pacman -S openssl pkg-config
+    pacman -S openssl-devel pkg-config
 
 and try building this crate again.