go/analysis/internal/checker: correct comment grammar

Change-Id: I78b41326c5d8d22fbff40f89721ac3aad3a5c63c
Reviewed-on: https://go-review.googlesource.com/c/158841
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

go/analysis/internal/checker/checker.go

@@ -42,7 +42,7 @@ var (
 	CPUProfile, MemProfile, Trace string
 )
 
-// RegisterFlags registers command-line flags used the analysis driver.
+// RegisterFlags registers command-line flags used by the analysis driver.
 func RegisterFlags() {
 	// When adding flags here, remember to update
 	// the list of suppressed flags in analysisflags.