all: fix a few function names on comments

Change-Id: I91eec68ebd9394685a6586c8d0cf01ecf3fd82e2
Reviewed-on: https://go-review.googlesource.com/c/tools/+/441775
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/internal/diff/ndiff.go b/internal/diff/ndiff.go
index 722b50c..4dd8323 100644
--- a/internal/diff/ndiff.go
+++ b/internal/diff/ndiff.go
@@ -91,7 +91,7 @@
 	return len
 }
 
-// isASCII reports whether s contains only ASCII.
+// stringIsASCII reports whether s contains only ASCII.
 // TODO(adonovan): combine when x/tools allows generics.
 func stringIsASCII(s string) bool {
 	for i := 0; i < len(s); i++ {