Clean up E0608 explanation

GuillaumeGomez · GuillaumeGomez · commit 632f3de12bc7 · 2020-05-25T13:03:38.000+02:00
diff --git a/src/librustc_error_codes/error_codes/E0608.md b/src/librustc_error_codes/error_codes/E0608.md
@@ -1,4 +1,4 @@
-An attempt to index into a type which doesn't implement the `std::ops::Index`
+An attempt to use index on a type which doesn't implement the `std::ops::Index`
 trait was performed.
 
 Erroneous code example:

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		-An attempt to index into a type which doesn't implement the `std::ops::Index`
	`1`	+An attempt to use index on a type which doesn't implement the `std::ops::Index`
`2`	`2`	`trait was performed.`
`3`	`3`
`4`	`4`	`Erroneous code example:`