Clean up E0723 explanation

GuillaumeGomez · GuillaumeGomez · commit 03bbe9d2c936 · 2020-07-16T17:04:34.000+02:00
diff --git a/src/librustc_error_codes/error_codes/E0723.md b/src/librustc_error_codes/error_codes/E0723.md
@@ -1,4 +1,4 @@
-An feature unstable in `const` contexts was used.
+An unstable feature in `const` contexts was used.
 
 Erroneous code example:
 

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		-An feature unstable in `const` contexts was used.
	`1`	+An unstable feature in `const` contexts was used.
`2`	`2`
`3`	`3`	`Erroneous code example:`
`4`	`4`