docs: fix article typos

a -> an

Change-Id: I6917aff4eb36ad1cd46b159391f39c1a70b54452
Reviewed-on: https://go-review.googlesource.com/63990
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/arm/armasm/ext_test.go b/arm/armasm/ext_test.go
index 98192b3..3556ae7 100644
--- a/arm/armasm/ext_test.go
+++ b/arm/armasm/ext_test.go
@@ -34,7 +34,7 @@
 	debug      = false
 )
 
-// A ExtInst represents a single decoded instruction parsed
+// An ExtInst represents a single decoded instruction parsed
 // from an external disassembler's output.
 type ExtInst struct {
 	addr uint32
diff --git a/arm/armasm/inst.go b/arm/armasm/inst.go
index 60d633b..0e05644 100644
--- a/arm/armasm/inst.go
+++ b/arm/armasm/inst.go
@@ -105,7 +105,7 @@
 	return fmt.Sprintf("#%#x", uint32(i))
 }
 
-// A ImmAlt is an alternate encoding of an integer constant.
+// An ImmAlt is an alternate encoding of an integer constant.
 type ImmAlt struct {
 	Val uint8
 	Rot uint8
diff --git a/ppc64/ppc64asm/ext_test.go b/ppc64/ppc64asm/ext_test.go
index b553984..95d4d46 100644
--- a/ppc64/ppc64asm/ext_test.go
+++ b/ppc64/ppc64asm/ext_test.go
@@ -35,7 +35,7 @@
 	debug      = false
 )
 
-// A ExtInst represents a single decoded instruction parsed
+// An ExtInst represents a single decoded instruction parsed
 // from an external disassembler's output.
 type ExtInst struct {
 	addr uint32
diff --git a/x86/x86asm/ext_test.go b/x86/x86asm/ext_test.go
index 7a088a0..72fe45b 100644
--- a/x86/x86asm/ext_test.go
+++ b/x86/x86asm/ext_test.go
@@ -33,7 +33,7 @@
 	debug      = false
 )
 
-// A ExtInst represents a single decoded instruction parsed
+// An ExtInst represents a single decoded instruction parsed
 // from an external disassembler's output.
 type ExtInst struct {
 	addr uint32