Tweak missing size error (#654)

Right now calls `SArray(rand(3,3,4))` produces this error. `SMatrix{3,3}` works but `SArray{3,3}` doesn't; this change makes the requirement of SArray{Tuple{3,3}} explicit.

Author: raminammour

src/traits.jl

@@ -78,8 +78,9 @@ function missing_size_error(::Type{SA}) where SA
         example, you might try
 
             m = zeros(3,3)
-            SMatrix(m)      # this error
-            SMatrix{3,3}(m) # correct - size is inferrable
+            SMatrix(m)            # this error
+            SMatrix{3,3}(m)       # correct - size is inferrable
+            SArray{Tuple{3,3}}(m) # correct, note Tuple{3,3}
         """)
 end