Merge pull request #810 from stan-dev/typo/uppers

fix typo in Reference Manual BNF syntax - "uppers" should be "upper"

src/reference-manual/types.qmd

@@ -1656,7 +1656,7 @@ dimensions `matrix[K, K]` types.
 +---------------------+-------------------------+----------------------------------------------+
 |                     |                         | `matrix[M, N]<upper=U>`                      |
 +---------------------+-------------------------+----------------------------------------------+
-|                     |                         | `matrix[M, N]<lower=L, uppers=U>`            |
+|                     |                         | `matrix[M, N]<lower=L, upper=U>`            |
 +---------------------+-------------------------+----------------------------------------------+
 |                     |                         | `matrix[M, N]<offset=O>`                     |
 +---------------------+-------------------------+----------------------------------------------+