Correcting typos

examples/hook/hook.go

@@ -1,7 +1,7 @@
 // +build ignore
-// Do NOT include the above line in your code. This is a build contrainst used
+// Do NOT include the above line in your code. This is a build constraint used
 // to prevent import loops in the code whilst go get'ting it.
-// Read more about build contrains in golang here:
+// Read more about build constraints in golang here:
 // https://golang.org/pkg/go/build/#hdr-Build_Constraints
 
 package main