Remove semicolon from text/plain content type in example Swagger specs (#2022)

The extra semicolon causes a `NonConformingResponseHeaders` exception
when response validation is enabled for the Swagger spec in these
examples.

Author: chrisinmtown

examples/frameworks/spec/swagger.yaml

@@ -13,7 +13,7 @@ paths:
       description: Generates a greeting message.
       operationId: post_greeting
       produces:
-        - text/plain;
+        - text/plain
       responses:
         '200':
           description: greeting response

examples/helloworld/spec/swagger.yaml

@@ -13,7 +13,7 @@ paths:
       description: Generates a greeting message.
       operationId: hello.post_greeting
       produces:
-        - text/plain;
+        - text/plain
       responses:
         '200':
           description: greeting response