Explain "a warning" will show, not "this warning"

We aren't showing what the warning is, so saying "this warning" is one
of those things that works better when you're talking than when you're
writing out instructions. By saying "this warning" I thought maybe I
was supposed to be able to see the warning itself in the text.

Author: ianjmacintosh

exercises/10.arrays/README.mdx

@@ -45,7 +45,7 @@ So we're interpolating an array of renderable elements. This is totally
 acceptable, but it has interesting implications for when things change over
 time.
 
-We haven't covered things changing over time yet, but this warning's going to
+We haven't covered things changing over time yet, but a warning's going to
 show up any time you render an array regardless of whether that array changes
 so we're going to address that here.