cmd/go/internal/semver: remove duplicate words in Compare doc

Change-Id: I1ab0b5f713581b5f497878f222fa4ba3998a0ccd
Reviewed-on: https://go-review.googlesource.com/c/go/+/196179Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/semver/semver.go

@@ -107,7 +107,7 @@ func Build(v string) string {
 }
 
 // Compare returns an integer comparing two versions according to
-// according to semantic version precedence.
+// semantic version precedence.
 // The result will be 0 if v == w, -1 if v < w, or +1 if v > w.
 //
 // An invalid semantic version string is considered less than a valid one.