A few more mermaid newline fixes (#5752)

Akin to #5751, found in two other files.

docs/design/expressions/arithmetic.md

@@ -58,9 +58,9 @@ standard library.
 %%{init: {'themeVariables': {'fontFamily': 'monospace'}}}%%
 graph BT
     negation["-x"]
-    multiplication>"x * y<br>
+    multiplication>"x * y
                     x / y"]
-    addition>"x + y<br>
+    addition>"x + y
               x - y"]
     modulo["x % y"]
 

docs/design/expressions/bitwise.md

@@ -59,7 +59,7 @@ graph BT
     bitwise_and>"x & y"]
     bitwise_or>"x | y"]
     bitwise_xor>"x ^ y"]
-    shift["x << y<br>
+    shift["x << y
            x >> y"]
     bitwise_and & bitwise_or & bitwise_xor & shift --> complement
 ```