database/sql: fix typo and wording

Clean up the phrasing a little bit, make the comment fit in 80
characters, and fix the spelling of "guard."

Change-Id: I688a3e760b8d67ea83830635f64dff04dd9a5911
Reviewed-on: https://go-review.googlesource.com/34792
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql_test.go

@@ -2607,13 +2607,13 @@ func TestIssue6081(t *testing.T) {
 	}
 }
 
-// TestIssue18429 attempts to stress rolling back the transaction from a context
+// TestIssue18429 attempts to stress rolling back the transaction from a
-// cancel while simultaneously calling Tx.Rollback. Rolling back from a context
+// context cancel while simultaneously calling Tx.Rollback. Rolling back from a
-// happens concurrently so tx.rollback and tx.Commit must gaurded to not
+// context happens concurrently so tx.rollback and tx.Commit must guard against
-// be entered twice.
+// double entry.
 //
-// The test is composed of a context that is canceled while the query is in process
+// In the test, a context is canceled while the query is in process so
-// so the internal rollback will run concurrently with the explicitly called
+// the internal rollback will run concurrently with the explicitly called
 // Tx.Rollback.
 func TestIssue18429(t *testing.T) {
 	db := newTestDB(t, "people")