Fix variable expansion regex in fish syntax (zyedidia#3830)

Author: remisalmon

runtime/syntax/fish.yaml

@@ -25,7 +25,7 @@ rules:
     - statement: "--[a-z-]+"
     - statement: "\\ -[a-z]+"
 
-    - identifier: "(?i)\\$\\{?[0-9A-Z_!@#$*?-]+\\}?"
+    - identifier: "(?i)\\{?\\$[0-9A-Z_!@#$*?-]+\\}?"
 
     - constant.string:
         start: "\""