remove redundant space

Author: bob-carpenter

src/reference-manual/types.qmd

@@ -707,7 +707,7 @@ sum_to_zero_vector[5] beta;
 ```
 
 Zero sum vectors are implemented as vectors and may be assigned to other
-vectors and vice-versa.  Zero sum  vector variables, like other constrained
+vectors and vice-versa.  Zero sum vector variables, like other constrained
 variables, are validated to ensure that they are indeed unit length; for
 zero sum  vectors, this is only done up to a statically specified accuracy
 threshold $\epsilon$ to account for errors arising from floating-point