tools/lint: Fix JSON lint pattern for spaces after ':'.

Previously, a string ending in "... 😄" was reported as an
error and the linter complained that there should be a space
after the last ':'. This commit changes the pattern so that the
linter only checks for colons that are preceded by an opening
double-quote (").

tools/linter_lib/custom_check.py

@@ -802,7 +802,7 @@ def build_custom_checkers(by_lang):
          'strip': '\n',
          'exclude': set(['zerver/webhooks/']),
          'description': 'Fix tab-based whitespace'},
-        {'pattern': r':["\[\{]',
+        {'pattern': r'":["\[\{]',
          'exclude': set(['zerver/webhooks/', 'zerver/tests/fixtures/']),
          'description': 'Require space after : in JSON'},
     ]  # type: RuleList