Correct typo in "emsdk help" message. (#959)

When invoking "emsdk help", there is a typo/grammatical error in the second to last paragraph.
See also:

    https://github.com/emscripten-core/emsdk/issues/957

emsdk.py

@@ -2926,7 +2926,7 @@ def main(args):
        be sure to match the same --build= option to both 'install' and
        'activate' commands and the invocation of 'emsdk_env', or otherwise
        these commands will default to operating on the default build type
-       which in and RelWithDebInfo.''')
+       which is RelWithDebInfo.''')
 
     print('''