spelling: expression

Signed-off-by: Josh Soref <[email protected]>

Author: jsoref

utils/gyb_syntax_support/ExprNodes.py

@@ -601,7 +601,7 @@
              Child('RightParen', kind='RightParenToken'),
          ]),
 
-    # postfix '#if' expession
+    # postfix '#if' expression
     Node('PostfixIfConfigExpr', kind='Expr',
          children=[
              Child('Base', kind='Expr', is_optional=True),