all: fix misuses of "a" vs "an"

Fixes the misuse of "a" vs "an", according to English grammatical
expectations and using https://www.a-or-an.com/

Change-Id: I53ac724070e3ff3d33c304483fe72c023c7cda47
Reviewed-on: https://go-review.googlesource.com/c/go/+/480536
Run-TryBot: shuang cui <imcusg@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

Author: cuishuang

scalar.go

@@ -104,7 +104,7 @@ func (s *Scalar) Set(x *Scalar) *Scalar {
 // If x is not of the right length, SetUniformBytes returns nil and an error,
 // and the receiver is unchanged.
 //
-// SetUniformBytes can be used to set s to an uniformly distributed value given
+// SetUniformBytes can be used to set s to a uniformly distributed value given
 // 64 uniformly distributed random bytes.
 func (s *Scalar) SetUniformBytes(x []byte) (*Scalar, error) {
 	if len(x) != 64 {