Fix expected error message in a test.

The change is a result of the char/string parsing refactor.

src/test/compile-fail/lex-unknown-str-escape.rs

@@ -9,5 +9,5 @@
 // except according to those terms.
 
 static s: &'static str =
-    "\●" //~ ERROR: unknown string escape
+    "\●" //~ ERROR: unknown character escape
 ;