cmd/compile: add missing quotation mark

Change-Id: I102901e3df76830ccd5ab74d757203d103eef9e8
Reviewed-on: https://go-review.googlesource.com/12657Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/doc.go

@@ -119,7 +119,7 @@ preempted.
 
 	//go:linkname localname importpath.name
 
-The //go:linkname directive instructs the compiler to use ``importpath.name' as the
+The //go:linkname directive instructs the compiler to use ``importpath.name'' as the
 object file symbol name for the variable or function declared as ``localname'' in the
 source code. Because this directive can subvert the type system and package
 modularity, it is only enabled in files that have imported "unsafe".