Small error message text change (remove comma after "But") (#6409)

Author: cknitt

jscomp/build_tests/super_errors/expected/partial_app.res.expected

@@ -7,7 +7,7 @@
   5 │ f(1, 2)
   6 │ 
 
-  This function call is at the top level and is expected to return `unit`. But, it's returning `int => int`.
+  This function call is at the top level and is expected to return `unit`. But it's returning `int => int`.
 
   In ReScript, anything at the top level must evaluate to `unit`. You can fix this by assigning the expression to a value, or piping it into the `ignore` function.
 

jscomp/build_tests/super_errors/expected/top_level_fn_call_not_unit.res.expected

@@ -7,7 +7,7 @@
   3 │ returnsSomething()
   4 │ 
 
-  This function call is at the top level and is expected to return `unit`. But, it's returning `int`.
+  This function call is at the top level and is expected to return `unit`. But it's returning `int`.
 
   In ReScript, anything at the top level must evaluate to `unit`. You can fix this by assigning the expression to a value, or piping it into the `ignore` function.
 

jscomp/build_tests/super_errors/expected/top_level_value_not_unit.res.expected

@@ -5,7 +5,7 @@
   1 │ 1234
   2 │ 
 
-  This is at the top level and is expected to return `unit`. But, it's returning `int`.
+  This is at the top level and is expected to return `unit`. But it's returning `int`.
 
   In ReScript, anything at the top level must evaluate to `unit`. You can fix this by assigning the expression to a value, or piping it into the `ignore` function.
 

jscomp/ext/warnings.ml

@@ -493,7 +493,7 @@ let message = function
       "Integer literal exceeds the range of representable integers of type int"
   | Bs_uninterpreted_delimiters s -> "Uninterpreted delimiters " ^ s
   | Bs_toplevel_expression_unit help ->
-      Printf.sprintf "This%sis at the top level and is expected to return `unit`. But, it's returning %s.\n\n  In ReScript, anything at the top level must evaluate to `unit`. You can fix this by assigning the expression to a value, or piping it into the `ignore` function.%s" 
+      Printf.sprintf "This%sis at the top level and is expected to return `unit`. But it's returning %s.\n\n  In ReScript, anything at the top level must evaluate to `unit`. You can fix this by assigning the expression to a value, or piping it into the `ignore` function.%s" 
         (match help with 
         | Some (_, FunctionCall) -> " function call " 
         | _ -> " ")