Fix windows test

Signed-off-by: Edward McFarlane <emcfarlane@buf.build>

cmd/protoc-gen-connect-go/main_test.go

@@ -180,7 +180,7 @@ func testCmpToTestdata(t *testing.T, content, path string) {
 	assert.Nil(t, err)
 	// Strip the copyright header and generated by line.
 	fileContent := string(b)
-	fileContent = fileContent[strings.Index(fileContent, "\n\n")+2:]
+	fileContent = fileContent[strings.Index(fileContent, "// Code generated by"):]
 	fileContent = strings.Replace(fileContent, "Code generated by protoc-gen-connect-go.", "Code generated by main.", 1)
 	assert.Zero(t, cmp.Diff(content, fileContent))
 }