Proposed fixes for Stan User's Guide

[yixuan]

Submission Checklist

• Builds locally
• New functions marked with <<{ since VERSION }>>
• Declare copyright holder and open-source license: see below

Summary

Two fixes for the "Matrices, Vectors, Arrays, and Tuples" section in Stan User's Guide:

• The original text had a clear misinterpretation of the data type tuple(array[5] int, vector[6]), which took it as a 3-tuple. The fix removes the non-existent int member in the tuple. Of course, another possibility is that the text corresponds to the data type tuple(array[5] int, int, vector[6]). I can modify my PR to reflect this possibility.
• There are some clear broken lines in the paragraph explaining aliasing, and I have to use my own understanding to complete the sentences. I'm also happy to modify my PR if my understanding is not 100% correct.

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

None. Just a few minor bug fixes.

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC BY-ND 4.0 (https://creativecommons.org/licenses/by-nd/4.0/)

[WardBrian]

Great catches, thank you!