Fix documentation of -list-[target-]features results

The manual claims that the -list-features and -list-target-features MI
commands return their result in a field named "result". The field is
actually named "features", and always has been since the introduction of
these commands in 084344d and c6ebd6c. See mi_cmd_list_features and
mi_cmd_list_target_features in gdb/mi/mi-main.c.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/doc/gdb.texinfo

@@ -38880,7 +38880,7 @@ Example output:
 
 @smallexample
 (gdb) -list-features
-^done,result=["feature1","feature2"]
+^done,features=["feature1","feature2"]
 @end smallexample
 
 The current list of features is:
@@ -38945,7 +38945,7 @@ Example output:
 
 @smallexample
 (gdb) -list-target-features
-^done,result=["async"]
+^done,features=["async"]
 @end smallexample
 
 The current list of features is: