More reliable function title in Gams

The function title regex is responsible for advancing the parser
but didn't pick up one-letter title, resulting in an infinite loop
for things like:

    I ..

Fixes #1505

src/languages/gams.js

@@ -149,7 +149,7 @@ function (hljs) {
         contains: [
               { // Function title
                 className: 'title',
-                begin: /^[a-z][a-z0-9_]+/,
+                begin: /^[a-z0-9_]+/,
               },
               PARAMS,
               SYMBOLS,