all: Fix comment typos

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GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4481

lib/model/rwfolder_test.go

@@ -386,7 +386,7 @@ func TestWeakHash(t *testing.T) {
 	}
 }
 
-// Test that updating a file removes it's old blocks from the blockmap
+// Test that updating a file removes its old blocks from the blockmap
 func TestCopierCleanup(t *testing.T) {
 	iterFn := func(folder, file string, index int32) bool {
 		return true