Fix the output of some examples in the "Case, cond and if chapter" (#729)

Author: whatyouhide

getting-started/case-cond-and-if.markdown

@@ -251,7 +251,7 @@ One thing to keep in mind when using `do/end` blocks is they are always bound to
 iex> is_number if true do
 ...>  1 + 2
 ...> end
-** (RuntimeError) undefined function: if/1
+** (CompileError) undefined function: is_number/2
 ```
 
 Would be parsed as:
@@ -260,10 +260,10 @@ Would be parsed as:
 iex> is_number(if true) do
 ...>  1 + 2
 ...> end
-** (RuntimeError) undefined function: if/1
+** (CompileError) undefined function: is_number/2
 ```
 
-Which leads to an undefined function error as Elixir attempts to invoke `if/1`. Adding explicit parentheses is enough to resolve the ambiguity:
+which leads to an undefined function error as Elixir attempts to invoke `is_number/1`, but passing it *two* arguments (the `if true` expression - which would throw an undefined function error itself as `if` needs a second argument, the `do/end` block - and the `do/end` block). Adding explicit parentheses is enough to resolve the ambiguity:
 
 ```iex
 iex> is_number(if true do