Fix defaultValue parsing

Currently vscode highlighting breaks for the entire file when a
graphql defaultValue is used. This is because we don't accept a
close paren as a possible terminator for the `graphql-variable-assignment`
token.

Replace the literal '.' with a closing paren.

Author: trevor-scheer

packages/vscode-apollo/syntaxes/graphql.json

@@ -267,7 +267,7 @@
     },
     "graphql-variable-assignment": {
       "begin": "\\s(=)",
-      "end": "(?=[\n.])",
+      "end": "(?=[\n)])",
       "applyEndPatternLast": 1,
       "beginCaptures": {
         "1": { "name": "punctuation.assignment.graphql" }